Troubleshooting faulty network connectivity, part 2: Essential network commands

In my previous article describing the troubleshooting steps for faulty TCP/IP connections, I mentioned several commands such as ping, traceroute, and ipconfig that could assist you in pinpointing problematic network components. These commands and several others like them are commonly referred to as TCP/IP utilities because they are tools that let you perform diagnostics and queries on the network which you are examining.
To compliment Part 1 of this two-part series, Part 2 is a reference list of the TCP/IP utilities which will describe the intended uses and options for each. These tools natively run in command line interface (CLI) environments (or in Linux and Unix, “shell prompts”), but as several entries in this blog show, there are a great many graphical utilities available that let you use the TCP/IP utilities (and view their output) in graphical format. In this article, however, I will stick with the default CLI usage and functionality.
When working in a CLI environment in Windows, I prefer to use Windows PowerShell as opposed to the default Windows command prompt (cmd.exe).
Remember: the best way to learn and understand these tools is to practice and experiment with them. Also bear in mind that knowledge of these commands is essential during ethical hacking efforts.

Jump to:

• arp
• hostname
• ifconfig
• ipconfig
• nbtstat
• netstat
• nslookup, dig, and host
• pathping
• ping
• route
• telnet
• tracert and traceroute
• whois

Arp

The arp command lets you view and manage the Address Resolution Protocol (ARP) cache. In other words, arp displays and modifies the IP address-to-MAC address translation tables used by the ARP protocol. In order for the arp command to be meaningful and helpful, you need to first understand the purpose of the Address Resolution Protocol. As DNS translates between host names and IP addresses, ARP translates between MAC addresses (Layer 2) and IP addresses (Layer 3). When a host attempts to communicate with another host on the same subnet, it must first know the destination host’s MAC address. If there is no entry in the sending host’s ARP cache for the destination MAC address, ARP sends out a broadcast (to all hosts in the subnet) asking the host with the target IP address to send back its MAC address. These IP-to-MAC mappings build up in the ARP cache which the arp command lets you view and modify.
Be aware that the ARP cache is a tempting target for hackers. It can be vulnerable to cache poisoning attacks in which false entries are inserted into the ARP cache, causing the compromised host to unknowingly send data (often unencrypted) to the attacker.
The default arp command syntax in Windows is:
arp -s inet_addr eth_addr [if_addr]
arp -d inet_addr [if_addr]
arp -a [inet_addr] [-N if_addr] [-v]

Arp command switches (Windows)	Description
arp -a or arp -g	Displays both the IP and MAC addresses in the ARP cache for all network interfaces using ARP.
arp -d [inet_addr]	Deletes all entries from the ARP cache which causes ARP queries for local network hosts to be re-processed. For example, arp -d 10.57.10.32.
arp -N [if_addr]	Displays the ARP entries for the network interface specified by [if_addr]. Used in conjunction with -a or -g. For example: arp -a -N 192.168.20.15, where 192.168.20.15 is the IP address of your (or one of your) network interfaces.
arp -s	Adds a static (“permanent”) entry to the ARP cache. This command is a countermeasure to ARP spoofing attacks. For example, this command adds a static entry: arp -s 157.55.85.212 00-aa-00-62-c6-09.
arp -v	Displays current ARP entries in verbose mode. All invalid entries and entries on the loopback interface will be shown. Used in conjunction with arp -a or arp -g.
[eth_addr]	Specifies a physical (MAC) address.
[if_addr]	If present, this specifies the Internet address of the interface on your computer whose address translation table should be modified. Useful if your computer has multiple network interfaces. If not present, the first applicable interface will be used.
[inet_addr]	Specifies an IP address entry in the ARP cache. Used in conjunction with -a or -g. For example, the command arp -a 192.168.10.20 will query the cache to display the MAC address of host 192.168. 10.20.

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The default arp command syntax in Linux is as follows:
arp [-evn] [-H type] [-i if] -a [hostname]
arp [-v] [-i if] -d hostname [pub]
arp [-v] [-H type] [-i if] -s hostname hw_addr [temp]
arp [-v] [-H type] [-i if] -s hostname hw_addr [netmask nm] pub
arp [-v] [-H type] [-i if] -Ds hostname ifa [netmask nm] pub
arp [-vnD] [-H type] [-i if] -f [filename]
Each complete entry in the ARP cache will be marked with the C (complete) flag. Permanent entries are marked with M (perManent) and published entries have the P (publish) flag.

Arp command switches (Linux)	Description
arp -a [hostname] or –all [hostname]	Shows the entries of the specified hosts. If the [hostname] parameter is not used, all entries will be displayed.
arp -d [ip_addr] or –delete [ip_addr]	Removes the ARP cache entry for the specified host. arp -d * will delete all hosts in the cache (note the space between -d and *).
arp -D or –use-device	Uses the hardware address associated with the specified interface.
arp -e	Shows the entries in default (Linux) style.
arp -f [filename] or –file [filename]	Similar to the -s option, only this time the address info is taken from file that [filename] set up. The name of the data file is very often /etc/ethers, but this is not official. If no [filename] is specified, /etc/ethers is used as default.
arp -H or –hw-type [type] or -t [type]	When setting or reading the ARP cache, this optional parameter tells arp which class of entries it should check for. The default value of this parameter is ether (i.e. hardware code 0×01 for IEEE 802.3 10Mbps Ethernet).
arp -i [int] or –device [int]	Selects an interface. When dumping the ARP cache only entries matching the specified interface will be printed. For example, arp -i eth0 -s 10.21.31.41 A321.ABCF.321A creates a static ARP entry associating IP address 10.21.31.41 with MAC address A321.ABCF.321A on eth0.
arp -n or –numeric	Shows IP addresses instead of trying to determine domain names.
arp -s [hostname] [hw_addr] or –set [hostname]	Manually creates a static ARP address mapping entry for host [hostname] with the hardware address set to [hw_addr].
arp -v	Uses verbose mode to provide more details.

Hostname

If you are unsure of the host name of the computer you are working on, the hostname command will display it. In Linux and Unix you can use hostname to configure the host name and associated options.

Hostname command (Windows)	Description
hostname	Prints the name of the current host where the name is the host name portion of the full computer name.

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The hostname command syntax in Linux is as follows:
hostname [-v] [-a] [-d] [-f] [-A] [-i] [-I] [--all-ip-addresses] [--long] [-s] [-y]
hostname [-v] [-b] [-F filename] [--file filename] [hostname]
hostname [-v] [-h] [-V]

Hostname command switches (Linux)	Description
hostname [hostname]	Sets the host name.
hostname -a or –alias	Displays the alias name of the host (if used).
hostname -A or –all-fqdns	Displays all FQDNs of the machine. This option enumerates all configured network addresses on all configured network interfaces, and translates them to DNS domain names. Addresses that cannot be translated (i.e. because they do not have an appropriate reverse DNS entry) are skipped.
hostname -b or –boot	Always set a hostname; this allows the file specified by -F to be non-existant or empty, in which case the default hostname localhost will be used if none is yet set.
hostname -d or –domain	Displays the DNS domain name.
hostname -f or –fqdn or –long	Displays the fully qualified domain name.
hostname -F [file] or –file [file]	Consults [file] for host name.
hostname -h or –help	Displays a help message.
hostname -i or–ip-addresses	Displays the host IP address.
hostname -I or –all-ip-addresses	Displays all network addresses of the host. This option enumerates all configured addresses on all network interfaces except the loopback interface and IPv6 link-local addresses. Unlike option -i, this option does not depend on name resolution.
hostname -s or –short	Trims domain information from the display output.
hostname -v or –verbose	Verbose mode
hostname -V or –version	Displays the version information and then exits.
hostname -y or –yp or -nis [domain name]	Displays the NIS domain name or sets a new NIS domain name with [domain name].

Ifconfig

The ifconfig command displays and configures the parameters for host network interfaces (e.g., IP address, subnet mask and default gateway) on Linux and Unix. Its Windows counterpart is ipconfig. When used without arguments, ifconfig displays the current configurations for all network interfaces (such as your NIC, wireless adapter, and loopback). Interface names are numbered starting at zero: eth0, eth1, wlan0, wlan1, etc.
The standard ifconfig command syntax is as follows:
ifconfig [interface or address_family type] options | address …

Ifconfig command switches	Description
ifconfig	Displays details on all network interfaces.
ifconfig [interface]	The name of the interface. This is usually a driver name followed by a unit number; for example, eth0 for the first Ethernet interface. Eth0 will usually be a PC’s primary network interface card (NIC).
ifconfig [address_family]	To enable the interpretation of differing naming schemes used by various protocols, [address_family] is used for decoding and displaying all protocol addresses. Currently supported address families include inet (TCP/IP, default), inet6 (IPv6), ax25 (AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) and netrom (AMPR Packet radio).
ifconfig [interface] add [address/prefixlength]	Add an IPv6 address to the [interface].
ifconfig [interface] address [address]	Assigns the specified IP [address] to the specified [interface].
ifconfig [interface] allmulti or -allmulti	Enables or disables all-multicast mode If selected, all multicast packets on the network will be received by the interface. This enables or disables the sending of incoming frames to the kernel’s network layer.
ifconfig [interface] arp or -arp	Enables or disables the use of the ARP protocol on this [interface].
ifconfig [interface] broadcast [address]	Specifies the address to use to use for broadcast transmissions. By default, the broadcast address for a subnet is the IP address with all ones in the host portion of the subnet address (i.e., a.b.c.255 for a /24 subnet).
ifconfig [interface] del [address/prefixlength]	Remove an IPv6 address from the [interface].
ifconfig [interface] down	Disables the [interface].
ifconfig [interface] hw [class] [address]	Sets the hardware (MAC) address of this [interface], if the device driver supports this operation. The keyword must be followed by the name of the hardware class and the printable ASCII equivalent of the hardware address. Hardware classes currently supported include ether (Ethernet), ax25 (AMPR AX.25), ARCnet and netrom (AMPR NET/ROM).
ifconfig [interface] io_addr [address]	Sets the start [address] in I/O space for this device.
ifconfig [interface] irq [address]	Sets the interrupt line used by the network interface.
ifconfig [interface] mem_start [address]	Sets the start address for shared memory of the interface.
ifconfig [interface] media [type]	Sets physical port or medium type. Examples of [type] are 10baseT, 10base2, and AUI. A [type] value of auto will tell the interface driver to automatically determine the media type (driver support for this command varies).
ifconfig [interface] mtu [n]	Sets the Maximum Transfer Unit (MTU) of an interface to [n].
ifconfig [interface] multicast	Sets the multicast flag on the interface (should not normally be needed as the drivers set the flag correctly themselves).
ifconfig [interface] netmask [mask_address]	Sets the IP subnet mask for this interface. This value defaults to the standard Class A, B, or C subnet masks (based on the interface IP address) but can be changed with this command.
ifconfig [interface] pointopoint or -pointopoint	Enables or disables point-to-point mode on this interface
ifconfig [interface] promisc or -promisc	Enables or disables promiscuous mode on the interface.
ifconfig [interface] txquelen [n]	Sets the transmit queue length on the interface. Smaller values are recommended for connections with high latency (i.e., dial-up modems, ISDN, etc).
ifconfig [interface] tunnel [address]	Creates a Simple Internet Transition (IPv6-inIPv4) device which tunnels to the IPv4 [address] provided.
ifconfig [interface] up	Activates (enables) the interface.

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For example, this command sets the IP address, subnet mask, and broadcast address for eth0:
ifconfig eth0 192.168.10.25 netmask 255.255.255.0 broadcast 192.168.10.255

Ipconfig

Ipconfig can display current TCP/IP network configuration values, update or release Dynamic Host Configuration Protocol (DHCP) allocated leases, and display, register, or flush Domain Name System (DNS) names. Its Linux/Unix counterpart is ifconfig.

ipconfig command switches	Description
ipconfig	Displays details on the active network interface.
ipconfig /all	Displays more details on both active and inactive network interfaces, including MAC address, DHCP server IP address, DNS server address(es), WINS server address(es), and whether or not NetBIOS Over TCP/IP is enabled.
ipconfig /allcompartments and /allcompartments /all	Shows information or detailed information (all) about all network compartments.
ipconfig /displaydns	Displays the contents of the DNS client resolver cache, which includes both entries preloaded from the local Hosts file and any recently obtained resource records for name queries. Similar to arp -a.
ipconfig /flushdns	Flushes (deletes) the contents of the DNS client resolver cache. Similar to arp -d.
ipconfig /registerdns	Initiates dynamic registration for the DNS names and IP addresses that are configured at a computer.
ipconfig /release and /release6 [adapter]	Releases (deletes) the dynamic (from DHCP) IPv4 or IPv6 parameters of all network interfaces (if no specific adapter name is specified. For example, ipconfig /release LO* releases only those interfaces starting with ‘Lo’).
ipconfig /renew and /renew6 [adapter]	Initiates communication with a DHCP server and renews the dynamic IPv4 or IPv6 parameters of all or some network interfaces. For example, ipconfig /renew LO* renews all interfaces starting with ‘Lo’.
ipconfig /setclassid and /setclassid6 [adapter]	Modifies the DHCP or DHCPv6 class id. Classes can be vendor-defined or user-defined and are implemented by the DHCP administrator when certain clients need different default gateway or DNS server parameters.
ipconfig /showclassid and /showclassid6 [adapter]	Displays the DHCP or DHCPv6 class id.

Nbtstat

Nbtstat is a Windows tool used to troubleshoot NetBIOS over TCP/IP name resolution problems. NetBIOS over TCP/IP is a networking protocol that allows legacy computer applications relying on the NetBIOS to be used on modern TCP/IP networks. Therefore, it is unlikely that NetBIOS over TCP/IP will be utilized within your network nowadays.
Nbtstat can check the state of current NetBIOS over TCP/IP (NetBT) connections, view and update the NetBIOS name cache, and determine the names registered with Windows Internet Name Service (WINS). If you are interested, nbtstat output is described on TechNet.
Do not confuse this command with netstat.
The standard nbtstat command syntax is:
nbtstat [ [-a RemoteName] [-A IP address] [-c] [-n] [-r] [-R] [-RR] [-s] [-S] [interval] ]

nbtstat command switches	Description
nbtstat -a [remote name]	Returns the NetBIOS name table and MAC address of the address card for the computer name specified.
nbtstat -A [remote IP address]	Lists the same information as nbtstat -a when given the target’s IP address.
nbtstat -c	Displays the contents of the NetBIOS name cache, the table of NetBIOS names, and their resolved IP addresses.
nbtstat -n	Displays the NetBIOS name table of the local computer. The status of ‘registered’ indicates that the name is registered either by broadcast or with a WINS server.
nbtstat -r	Lists names resolved by broadcast and via WINS.
nbtstat -R	Purges and reloads the remote cache name table.
nbtstat -RR	Releases and then refreshes NetBIOS names for the local computer that is registered with WINS servers.
nbtstat -s	Displays NetBIOS client and server sessions, attempting to convert the destination IP address to a name.
nbtstat -S	Displays NetBIOS client and server sessions, listing the remote computers by destination IP address only.
[interval]	Redisplays selected statistics, pausing every [interval] seconds between each display. Press Ctrl+C to stop redisplaying statistics.

Netstat

The netstat command displays the TCP/IP protocol statistics and active connections on the computer on which it was executed. Netstat is particularly useful when you suspect that there may be unauthorized connections to your computer (such as when a possible malware has occurred).Two popular graphical viewers for netstat are TCPEye and CurrPorts.
Netstat can be used to detect SYN floods that may be affecting a host. If you run a netstat command such as netstat -n -p TCP and you see many connections in the SYN_RECV state, you know some anomaly is occurring.
Do not confuse this command with nbtstat.
The standard netstat command syntax in Windows is:
netstat [-a] [-b] [-e] [-f] [-n] [-o] [-p proto] [-r] [-s] [-t] [interval]

Netstat command switches (Windows)	Description
netstat	Shows the active connections for all outbound TCP/IP connections.
netstat -a	Displays a more comprehensive list of active connections and the ports on which the computer is listening (includes UDP).
netstat -b	Displays the executable involved in creating each connection or listening port.
netstat -e	Displays Ethernet-related statistics.
netstat -f	Displays Fully Qualified Domain Names (FQDN) for foreign addresses. With this option you can check if your PC is connected to suspicious websites.
netstat -n	Displays active TCP connections; addresses and port numbers are expressed numerically; no attempt is made to determine host names.
netstat -o	Displays the owning process ID (PID) associated with each connection. You can look up a PID with the Windows Task Manager.
netstat -p [proto]	Displays connection details for only a certain protocol, where [proto] can be TCP, UDP, TCPv6, or UDPv6. With the additional -s option, [proto] can be IP, IPv6, ICMP, ICMPv6, TCP, TCPv6, UDP, or UDPv6.
netstat -r	Displays the host’s routing table.
netstat -s	Displays per-protocol statistics. By default, statistics are shown for IP, IPv6, ICMP, ICMPv6, TCP, TCPv6, UDP, and UDPv6; the -p option may be used to specify a subset of the default.
netstat -t	Displays the current connection offload state.
[interval]	Specifies the length of time in seconds to wait before displaying fresh statistics.

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Netstat command switches (Linux)	Description
netstat -a or –all	Shows both listening and non-listening sockets.
netstat -A [family] or –protocol=[family]	Specifies the address families for which connections are to be shown. [family] is a comma separated list of address family keywords like inet, unix, ipx, ax25, netrom, and ddp. This has the same effect as using the –inet, –unix (-x), –ipx, –ax25, –netrom, and –ddp options.
netstat -c or –continuous	Configures netstat to refresh the displayed information every second until stopped.
netstat -C	Prints routing information from the route cache.
netstat -e or –extend	Displays an increased level of detail. Can be entered as twice (as –ee) for maximum details.
netstat -F	Prints routing information from the forward information database (FIB).
netstat -g or –groups	Displays multicast group membership information for IPv4 and IPv6.
netstat -i or –interface=[name]	Displays a table of all network interfaces, or the specified [name].
netstat -l or –listening	Shows only listening sockets (which are omitted by netstat be default).
netstat -M or –masquerade	Displays a list of masqueraded connections (connections being altered by Network Address Translation).
netstat -n or –numeric	Show numerical addresses instead of trying to determine symbolic host, port or user names (skips DNS translation).
netstat –numeric-hosts	Shows numerical host addresses but does not affect the resolution of port or user names.
netstat –numeric ports	Shows numerical port numbers but does not affect the resolution of host or user names.
netstat –numeric-users	Shows numerical user IDs but does not affect the resolution of host or port names.
netstat -N or –symbolic	Displays the symbolic host, port, or user names instead of numerical representations. Netstat does this by default.
netstat -o or –timers	Includes information related to networking timers.
netstat -p or –program	Shows the process ID (PID) and name of the program to which each socket belongs.
netstat -r or –route	Shows the kernel routing tables.
netstat -s or –statistics	Displays summary statistics for each protocol.
netstat -t or –tcp	Filters results to display TCP only.
netstat -T or –notrim	Stops trimming long addresses.
netstat -u or –udp	Filters results to display UDP only.
netstat -v or –verbose	Produces verbose output.
netstat -w or –raw	Filter results to display raw sockets only.
netstat -Z or –context	Prints the SELinux context if SELinux is enabled. On hosts running SELinux, all processes and files are labeled in a way that represents security-relevant information. This information is called the SELinux context.

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Some common netstat commands are: netstat -natp, netstat -t -listening, netstat -ntu, netstat -nap, netstat -ntulp, netstat -rne, and netstat -anp | grep [port_number]. Using the table above you can figure out what each of these commands does.

Nslookup, Dig, and Host

NSlookup (“name server lookup”) is a DNS query tool. This means that nslookup checks DNS records, domain host aliases, domain host services, and operating system information by querying DNS servers. Nslookup can also be used to perform DNS zone transfers and is useful when performing network footprinting during ethical hacking efforts. It has two modes: interactive and noninteractive. Interactive mode queries DNS servers for details about various hosts and domains. Noninteractive mode prints only the name and requested information for a host or domain.
Although it is still available by default on Windows and Linux/Unix, nslookup has been deprecated and further use is discouraged. It has effectively been replaced by its successors, the dig (Domain Information Groper) and host utilities. Unlike nslookup, they are not available natively on Windows and must be installed manually. There is a host command in Windows PowerShell but that is something different. You can install the Windows versions of dig and host by extracting them from BIND for Windows available here.
Dig is basically an improved version of nslookup. Host enables quick lookups of DNS server information and is used to find 1) the IP address of a given domain name and 2) the domain name of a given IP address.

Nslookup command switches (Windows)	Description
C:\>nslookup	Enters interactive mode; prompt changes from C:> to >.
C:\>nslookup – [DNSserver]	Enters interactive mode using a particular DNS server.
C:\>nslookup [hostname] or [IPaddress]	Queries DNS for the IP address of a particular host or vice versa.
C:\>nslookup [hostname or IPaddress] [DNSserver]	Queries DNS for the IP address of a particular host (or vice versa) using a particular DNS server.
C:>nslookup -type=[type]	Queries for a particular type of DNS records, where [type] can be: A, ANY, CNAME, GID, HINFO, MB, MG, MINFO, MR, MX, NS, PTR, SOA, TXT, UID, UINFO, WKS. DNS record types are described here and here.
C:>nslookup -type=MX [example.com]	Queries for email records for domain example.com
>exit	Exits interactive mode (back to noninteractive mode).
>finger	Connects with the finger server on the current computer.
>help	Displays a short summary of subcommands.
>ls [example.com]	Lists information for a DNS domain.
>ls -a [example.com]	Lists canonical names and aliases.
>ls -d [example.com]	Lists all DNS records for example.com. Can be used to launch a zone transfer when connected to the SOA for a DNS zone. These transfers can be blocked in the DNS properties of the server.
>ls -t [type] [example.com]	Lists records of the given RFC record [type] (e.g., A ,CNAME, MX, NS, PTR, etc.)
>lserver	Changes the default server to the specified DNS domain.
>root	Changes the default server to the server for the root of the DNS domain name space.
>server	Changes the default server to the specified DNS domain.
>set	Changes configuration settings that affect how lookups function.
>set all	Prints the current values of the configuration settings.
>set class=X	Changes the query class. The class specifies the protocol group of the information.
>set [no]d2	Turns exhaustive Debugging Mode on or off. All fields of every packet are printed.
>set [no]debug	Turns Debugging Mode on or off.
>set [no]defname	Appends the default DNS domain name to a single component lookup request. A single component is a component that contains no periods.
>set domain=NAME	Changes the default DNS domain name to the name specified.
>set ignore	Ignores packet truncation errors.
>set ixfrver=X	Current version to use in IXFR transfer request.
>set [no]msxfr	Use MS fast zone transfer.
>set port	Changes the default TCP/UDP DNS name server port to the value specified.
>set querytype=X	Changes the resource record type for the query.
>set [no]recurse	Tells the DNS name server to query other servers if it does not have the information.
>set retry=X	Sets the number of retries to X.
>set root=NAME	Changes the name of the root server used for queries.
>set [no]search	Appends the DNS domain names in the DNS domain search list to the request until an answer is received. This applies when the set and the lookup request contain at least one period, but do not end with a trailing period.
>set srchlist=N1[/N2/.../N6]	Changes the default DNS domain name to N1 and search list.
>set timeout=X	Changes the initial number of seconds to wait for a reply to a request.
>set type=X	Changes the resource record type for the query.
>set [no]vc	Specifies to use or not use a virtual circuit when sending requests to the server.
>set view	Sorts and lists the output of the previous ls subcommand or commands.
>view	Sorts and lists the output of the previous ls subcommand or commands.

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The default nslookup command syntax in Linux is:
nslookup [-option] [name | -] [server]

Nslookup command switches (Linux)	Description
$ nslookup	Enters interactive mode; prompt changes from $ to >.
$ nslookup [hostname or IP address]	Queries DNS for the IP address of a particular host or vice versa.
>exit	Exits nslookup.
>host [server]	Looks up information for host using the current default server or using server, if specified. If host is an Internet address and the query type is A or PTR, the name of the host is returned. If host is a name and does not have a trailing period, the search list is used to qualify the name.
>server [domain] or lserver [domain]	Changes the default server to [domain]; lserver uses the initial server to look up information about [domain], while server uses the current default server. If an authoritative answer can’t be found, the names of servers that might have the answer are returned.
>set	This command is used to change state information that affects the lookups.
>set all	Prints the current values of the frequently used options to set. Information about the current default server and host is also printed.
>set class=[value]	Changes the query class to [value].
>set class=IN	Changes the query class to the Internet class.
>set class=CH	Changes the query class to the Chaos class.
>set class=HS	Changes the query class to the Hesiod class.
>set class=ANY	Changes the query class to wildcard.
>set debug or nodebug	Turn on or off the display of the full response packet and any intermediate response packets when searching. (Default = nodebug; abbreviation = [no]deb)
>set d2 or nod2	Turns debugging mode on or off. This displays more about what nslookup is doing. (Default = nod2)
>set domain=[name]	Sets the search list to [name].
>set search or nosearch	If the lookup request contains at least one period but doesn’t end with a trailing period, appends the domain names in the domain search list to the request until an answer is received. (Default = search)
>set port=[value]	Changes the default TCP/UDP name server port to [value]. (Default = 53; abbreviation = po)
>set querytype=[value]	Changes the type of the information query. (Default = A; abbreviations = q, ty)
>set type=[value]	Changes the type of the information query. (Default = A; abbreviations = q, ty)
>set recurse or no recurse	Tells the DNS server to query other servers if it does not have the information. (Default = recurse; abbreviation = [no]rec)
>set retry=[number]	Sets the [number] of retries to attempt.
>set timeout=[number]	Changes the initial timeout interval for waiting for a reply to number seconds.
>set vc or novc	Always use a virtual circuit when sending requests to the server. (Default = novc)
>set fail or nofail	Tries the next nameserver if a DNS server responds with SERVFAIL or a referral (nofail) or terminate query (fail) on such a response. (Default = nofail).

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The standard dig command syntax is as follows:
dig [@server] [options] [name] [type] [class] [+queryopt...]

Dig command switches	Description
dig	Displays the DNS root servers and the the IP address of your host’s DNS server.
@[server]	The name or IP address of the DNS server to query. If no [server] argument is provided, dig consults /etc/resolv.conf and queries the DNS server(s) listed there. The reply from the DNS server that responds is displayed in the command output.
[name]	The domain name to query.
[type]	Indicates which type of query is required, such as ANY, A, MX, SIG, etc. If no [type] argument is supplied, dig will perform a lookup for an A record.
dig -b [address]	Sets the source IP address of the DNS query to [address]. This must be a valid address on one of the host’s network interfaces or “0.0.0.0″ or “::”. An optional port may be specified by appending “#”.
dig -c [class]	Sets the query [class] (described in the nslookup for Linux options above). Default is IN (Internet).
dig -f [filename]	Configures dig to process the list of lookup requests in file specified.
dig -h	Prints a brief summary of dig’s command-line arguments and options.
dig -k [TSIG_key_file]	Signs the DNS queries sent by dig and their responses using transaction signatures (TSIG).
dig -m	Enables memory usage debugging.
dig -p [portnumber]	Configures the port number for dig to query. Of course, the default port is 53 for DNS.
dig -q [name]	Sets the query name to the [name] parameter value specified.
dig -t [type]	Sets the [type] of query to be performed. Default is A.
dig -x [addr]	Used for reverse lookups (IP address to host name, rather than vice versa).
dig -y [hmac:]name:key]	Signs DNS queries by specifying the TSIG key itself on the command line. hmac is the type of the TSIG, default HMAC-MD5, [name] is the name of the TSIG key and [key] is the actual key. The key is a base-64 encoded string, typically generated by dnssec-keygen.
dig +aaflag or +noaaflag	A synonym for +[no]aaonly.
dig +aaonly or +noaaonly	Sets or removes the “aa” flag in the query.
dig +additional or +noadditional	Displays or does not display the additional section of a reply. The default is to display it.
dig +adflag or +noadflag	Set or removes the AD (authentic data) bit in the query. This requests the DNS server to return whether all of the answer and authority sections have all been validated as secure according to the security policy of the server. AD=1 indicates that all records have been validated as secure and the answer is not from an OPT-OUT range. AD=0 indicates that some part of the answer was insecure or not validated.
dig +all or +noall	Sets or clears all display flags.
dig +answer or +noanswer	Displays or does not display the answer section of a reply. The default is to display it.
dig +authority or +noauthority	Displays or does not display the authority section of a reply. The default is to display it.
dig +besteffort or +nobesteffort	Attempts to display the contents of messages which are malformed. The default is to not display malformed answers.
dig +bufsize=[B]	Sets the UDP message buffer size advertised using EDNS0 to [B] bytes. The maximum and minimum sizes of this buffer are 65535 and 0 respectively. Values outside this range are rounded up or down appropriately. Values other than zero will cause an EDNS query to be sent.
dig +edns=# or +noedns	Specifies the EDNS version to query with. Valid values are 0 to 255. Setting the EDNS version will cause a EDNS query to be sent. +noedns clears the remembered EDNS version.
dig +cdflag or +nocdflag	Sets or removes the CD (checking disabled) bit in the query. This requests the server to not perform DNSSEC validation of responses.
dig +cl or +nocl	Display or does not display the CLASS when printing the record.
dig +cmd or +nocmd	Toggles or untoggles the printing of the initial comment in the output identifying the version of dig and the query options that have been applied. This comment is printed by default.
dig +comments or +nocomments	Toggle or untoggles the display of comment lines in the output. The default is to print comments.
dig +defname or +nodefname	Deprecated; treated as a synonym for +[no]search.
dig +domainname=[somename]	Set the search list to contain the single domain [somename], as if specified in a domain directive in /etc/resolv.conf, and enable search list processing as if the +search option were given.
dig +dnssec or +nodnssec	Requests DNSSEC records be sent or not sent by setting the DNSSEC OK bit (DO) in the OPT record in the additional section of the query.
dig +fail or +nofail	Try or do not try the next server if you receive a SERVFAIL. The default is to not try the next server which is the reverse of normal stub resolver behavior.
dig +identify or +noidentify	Shows or does not show the IP address and port number that supplied the answer when the +short option is enabled. If short form answers are requested, the default is not to show the source address and port number of the server that provided the answer.
dig +ignore or +noignore	Ignores or does not ignore truncation in UDP responses instead of retrying with TCP. By default, TCP retries are performed.
dig +multiline or +nomultiline	Prints or does not print records like the SOA records in a verbose multi-line format with human-readable comments. The default is to print each record on a single line, to facilitate machine parsing of the dig output.
dig +ndots=[D]	Sets the number of dots that have to appear in name to [D] for it to be considered absolute. The default value is that defined using the ndots statement in /etc/resolv.conf, or 1 if no ndots statement is present. Names with fewer dots are interpreted as relative names and will be searched for in the domains listed in the search or domain directive in /etc/resolv.conf.
dig +nsid or +nonsid	Include or exclude an EDNS name server ID request when sending a query.
dig +nssearch or +nonssearch	When this option is set, dig attempts to 1) find the authoritative name servers for the zone containing the name being looked up and 2) display the SOA record that each name server has for the zone.
dig +onesoa or +noonesoa	Print only one (starting) SOA record when performing an AXFR. The default is to print both the starting and ending SOA records.
dig +qr or +noqr	Prints or does not print the query as it is sent. By default, the query is not printed.
dig +question or +noquestion	Prints or does not print the question section of a query when an answer is returned. The default is to print the question section as a comment.
dig +retry=[T]	Sets the number of times to retry UDP queries to server to [T] instead of the default, 2. Unlike +tries, this does not include the initial query.
dig +search or +nosearch	Uses or does not use the search list defined by the searchlist or domain directive in resolv.conf (if any). The search list is not used by default.
dig +short or +noshort	Provides or does not provide a terse (brief) answer. The default is to print the answer in a verbose form.
dig +showsearch or +noshowsearch	Performs or does not perform a search showing intermediate results.
dig +sigchase or +nosigchase	Chases or does not chase DNSSEC signature chains. Requires dig be compiled with -DDIG_SIGCHASE.
dig +stats or +nostats	This query option toggles or untoggles the printing of statistics; i.e., when the query was made, the size of the reply, and so on. The default behavior is to print the query statistics.
dig +tcp or +notcp	Uses or does not use TCP when querying name servers. The default behavior is to use UDP unless an AXFR (full) or IXFR (incremental) query is requested, in which case a TCP connection is used.
dig +time=[T]	Sets the timeout for a query to [T] seconds. The default timeout is 5 seconds. An attempt to set [T] to less than 1 will result in a query timeout of 1 second being applied.
dig +ttlid or +nottlid	Displays or does not display the TTL when printing the record.
dig +topdown or +notopdown	When chasing DNSSEC signature chains, performs or does not perform a top-down validation. Requires dig be compiled with -DDIG_SIGCHASE.
dig +trace or +notrace	Toggle or untoggles tracing of the delegation path from the root name servers for the name being looked up. Tracing is disabled by default. When tracing is enabled, dig makes iterative queries to resolve the name being looked up. It will follow referrals from the root servers, showing the answer from each server that was used to resolve the lookup
dig +tries=[T]	Sets the number of times to try UDP queries to server to [T] instead of the default, 3. If [T] is less than or equal to zero, the number of tries is silently rounded up to 1.
dig +trusted-key=####	Specifies a file containing trusted keys to be used with +sigchase. Each DNSKEY record must be on its own line. If not specified, dig will look for /etc/trusted-key.key, then trusted-key.key in the current directory. Requires dig be compiled with -DDIG_SIGCHASE.
dig +vc or +novc	Uses or does not use TCP when querying name servers. This alternate syntax to +[no]tcp is provided for backwards compatibility. The “vc” stands for virtual circuit.

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The standard host command syntax is as follows:
host [-aCdlnrsTwv] [-c class] [-N ndots] [-R number] [-t type] [-W wait] [-m flag] [-4] [-6] {name} [server]

Host command switches	Description
host -a	Equivalent to setting the -v option and asking host to make a query of [type] ANY.
host -c [class]	Specifies the DNS query class (described in the nslookup for Linux options above). Default is IN (Internet).
host -C [zone_name]	Attempts to display the SOA records for [zone_name] from all the listed authoritative name servers for that zone. The list of name servers is defined by the NS records that are found for the zone.
host -d or -v	Enables verbose output.
host -i	Specifies that reverse lookups of IPv6 addresses should use the IP6.INT domain as defined in RFC 1886. The default is to use IP6.ARPA.
host -l [zone_name]	Enables list mode which makes host perform a zone transfer for [zone_name]. Transfer the zone printing out the NS, PTR and address records (A/AAAA). If combined with -a all records will be printed.
host -m [zone_name]	Used to set the memory usage debugging flags [record], [usage] and [trace].
host -N [name]	Sets the number of dots that have to be in name for it to be considered absolute. The default value is that defined using the ndots statement in /etc/resolv.conf, or 1 if no ndots statement is present. Names with fewer dots are interpreted as relative names and will be searched for in the domains listed in the search or domain directive in /etc/resolv.conf.
host -r [name]	Specifies non-recursive queries. Setting this option clears the RD (recursion desired) bit in the query which host makes. This should mean that the name server receiving the query will not attempt to resolve [name]. The -r option enables host to mimic the behavior of a DNS server by making non-recursive queries and expecting to receive answers to those queries that are usually referrals to other DNS servers.
host -R [number]	Changes the number of UDP retries for a lookup.
host -s	Used to set the memory usage debugging flags [record], [usage] and [trace].
host -t [type]	Selects the DNS query type, where [type] can be CNAME, NS, SOA, SIG, KEY, AXFR, etc. When no query [type] is specified, host automatically selects an appropriate query type. By default, it looks for A, AAAA, and MX records, but if the -C option was given, queries will be made for SOA records, and if [name] is a dotted-decimal IPv4 address or colon-delimited IPv6 address, host will query for PTR records.
host -T	Makes host use a TCP connection when querying a DNS server.
host -w	Configures host to effectively wait forever for a reply.
host -W [wait]	Makes host wait for [wait] seconds.
host -4	Forces host to only use IPv4 query transport.
host -6	Forces host to only use IPv6 query transport.

PathPing

PathPing is a Windows utility that combines features from ping and tracert. PathPing sends packets to each router (hop) on the way to a destination over a period of time, and then computes the packets lost and the performance statistics for each hop. Like tracert, PathPing can identify problematic network hosts. The destination can be specified by DNS name or IP address.
You can download a neat little PathPing desktop gadget (for Windows Vista or 7) from TechRepublic here. You’ll have to rename the PathPing.zip file to PathPing.gadget.
Two PathPing switches are apparently no longer supported on Windows 7: -R (RSVP test) and -T (Layer Two tag).
The standard PathPing command syntax is:
pathping [-g host-list] [-h max_hops] [-i address] [-n] [-p period] [-q num_queries] [-w timeout] [-4] [-6] target_name

PathPing command switches	Description
pathping -g	Loose source route along host list.
pathping -h [number]	Maximum [number] of hops to search for target.
pathping -i [address]	Use the specified source [address].
pathping -n	Do not resolve addresses to hostnames.
pathping -p [milliseconds]	Wait period [milliseconds] between pings.
pathping -q [number]	[Number] of queries per hop.
pathping -w [milliseconds]	Wait timeout in [milliseconds] for each reply.
pathping -4	Force using IPv4.
pathping -6	Force using IPv4.

Ping

The ping (Packet InterNet Groper) command is arguably the most useful networking troubleshooting utility; it is definitely the simplest and most used. Ping tests the online status of a host on an IP network and measures the total round-trip time (in milliseconds) for packets sent from the source host to a destination host and back. Ping does this by sending Internet Control Message Protocol (ICMP) packets (usually). ICMP is a protocol that works with IP to provide error checking and reporting functionality. Ping sends ICMP echo requests to a remote host. If the host is able to respond, it replies with echo reply.
When you ping a destination name, the replies will tell you the host’s IP address, the number of bytes sent, round-trip time, and the packets’ Time to Live (TTL). When you ping a destination IP address, it responds with all the above except for the host name (unless you provide the -a switch). Ping doesn’t just test connectivity – it can also verify that TCP/IP is installed correctly and that DNS name resolution is working properly (see Part 1 of this blog post).
Standard ICMP pings do not utilize port numbers or use TCP or UDP; ICMP is a Layer 3 (Network layer) protocol. TCP and UDP operate at Layer 4, the Transport layer.
The ping command can be misused in a variety of denial of service attacks, such as the Ping of Death, Smurf attack, and Ping flood.
In Windows the ping command follows this standard syntax:
ping [-t] [-a] [-n count] [-l size] [-f] [-i TTL] [-v ToS] [-r count] [-s count] [[-j host-list] | [-k host-list]] [-w timeout] [-R] [-S srcaddr] [-4] [-6] target_name

Ping command switches (Windows)	Description
ping -a	Specifies that reverse name resolution is performed on the destination IP address.
ping -f	Specifies that Echo Request messages are sent with the ‘Don’t Fragment’ flag in the IP header set to 1 (available on IPv4 only).
ping -i [TTL]	Specifies the value of the TTL field in the IP header for Echo Request messages sent. The default is the default TTL value for the host. The maximum [TTL] is 255.
ping -j [hostlist]	Specifies that the Echo Request messages use the Loose Source Route option in the IP header with the set of intermediate destinations specified in [HostList] (available on IPv4 only). With loose source routing, successive intermediate destinations can be separated by one or multiple routers. The maximum number of addresses or names in the host list is 9. The host list is a series of IP addresses (in dotted decimal notation) separated by spaces.
ping -k [hostlist]	Specifies that the Echo Request messages use the Strict Source Route option in the IP header with the set of intermediate destinations specified in [HostList] (available on IPv4 only). With strict source routing, the next intermediate destination must be directly reachable (it must be a neighbor on an interface of the router). The maximum number of addresses or names in the host list is 9. The host list is a series of IP addresses (in dotted decimal notation) separated by spaces.
ping -l [size]	Specifies the length, in bytes, of the Data field in the Echo Request messages sent. The default size is 32. The maximum size is 65,527.
ping -n [count]	Specifies the number of Echo Request messages sent. The default is 4.
ping -r [count]	Specifies that the Record Route option in the IP header is used to record the path taken by the Echo Request message and corresponding Echo Reply message (available on IPv4 only). Each hop in the path uses an entry in the Record Route option. If possible, specify a [count] that is equal to or greater than the number of hops between the source and destination. The [count] must be a minimum of 1 and a maximum of 9.
ping -R	Specifies that the round-trip path is traced (IPv6 only).
ping -s [count]	Specifies that the Internet Timestamp option in the IP header is used to record the time of arrival for the Echo Request message and corresponding Echo Reply message for each hop. The [count] must be a minimum of 1 and a maximum of 4. This is required for link-local destination addresses.
ping -S [SrcAddr]	Specifies the source address to use (IPv6 only).
ping -t	Specifies that ping continue sending Echo Request messages to the destination until interrupted. To interrupt and display statistics, press CTRL+BREAK. To interrupt and quit ping, press CTRL+C.
ping -v [ToS]	Specifies the value of the Type of Service [ToS] field in the IP header for Echo Request messages sent (available on IPv4 only). The default is 0. [ToS] is specified as a decimal value from 0 through 255.
ping -w [timeout]	Specifies the amount of time, in milliseconds, to wait for the Echo Reply message that corresponds to a given Echo Request message to be received. If the Echo Reply message is not received within the time-out, the “Request timed out” error message is displayed. The default [timeout] is 4000 (4 seconds).
ping -4	Specifies that IPv4 is used to ping. This parameter is not required to identify the target host with an IPv4 address. It is only required to identify the target host by name.
ping -6	Specifies that IPv6 is used to ping. This parameter is not required to identify the target host with an IPv6 address. It is only required to identify the target host by name.

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Ping command switches (Linux)	Description
ping -a	Audible ping.
ping -A	Adaptive ping (interpacket interval adapts to round-trip time).
ping -b	For pinging a broadcast address.
ping -B	Does not allow ping to change the source address of probes. The address is bound to the one selected when ping starts.
ping -c [count] [deadline]	Stops after sending [count] ECHO_REQUEST packets. With [deadline] option, ping waits for [count] ECHO_REPLY packets, until the timeout expires.
ping -d	Sets the SO_DEBUG option on the socket being used. Essentially, this socket option is not used by Linux kernel.
ping -D	Prints the timestamp (Unix time + microseconds, as in gettimeofday) before each line.
ping -f	For every ECHO_REQUEST sent, a period (.) is printed, while for every ECHO_REPLY received a backspace is printed. This provides a rapid display of how many packets are being dropped.
ping -F	Only for ping6. Allocates and sets a 20-bit flow label on ECHO_REQUEST packets. If the value is zero, the kernel allocates a random flow label.
ping -i [interval]	Wait [interval] seconds between sending each packet. The default is to wait for one second between each packet normally, or not to wait in flood mode (-f).
ping -I [interface]	Sets the source address to the specified interface, whether numeric IP address or name of device. When pinging IPv6 link-local addresses this option is required.
ping -l [preload]	Sends [preload] number of packets and does not wait for replies. Only the super-user may select a [preload] of more than 3.
ping -L	Suppress loopback of multicast packets (only applies if the ping destination is a multicast address).
ping -m	Uses [mark] to tag the packets going out.
ping -M [hint]	Selects path MTU discovery strategy. [Hint] may be either do (prohibit fragmentation, even local one), want (do PMTU discovery, fragment locally when packet size is large), or dont (do not set DF flag).
ping -n	Specifies numeric output only. No attempt is made to look up symbolic names for IP addresses.
ping ->N	Sends ICMPv6 Node Information Queries (RFC 4620), instead of Echo Request.
ping -p [pattern]	Specifies up to 16 pad bytes (in hex) to fill out the packet you send. For example, -p ff will cause the sent packet to be filled with all ones.
ping -q	Quiet output. Nothing is displayed except the summary lines at command startup time and when finished.
ping -Q [tos]	Sets Quality of Service-related bits in ICMP datagrams. [Tos] can be either decimal or hex number.
ping -r	Bypasses the normal routing tables and sends directly to a host on an attached interface. If the host is not on a directly attached network, an error is returned.
ping -R	Records the route. Includes the RECORD_ROUTE option in the ECHO_REQUEST packet and displays the route buffer on returned packets. Note that the IP header is only large enough for nine such routes. Many hosts ignore or discard this option.
ping -s [packet_size]	Specifies the number of data bytes to be sent. The default is 56, which translates into 64 ICMP data bytes when combined with the 8 bytes of ICMP header data.
ping -S [sndbuf]	Set the socket send buffer. If not specified, it is selected to buffer not more than one packet.
ping -t [TTL]	Sets the IP TTL to [TTL] seconds.
ping -T [timestamp] [opt]	Sets special IP timestamp options. [Timestamp] option may be either tsonly (only timestamps), tsandaddr (timestamps and addresses) or tsprespec host1…
ping -U	Prints full user-to-user latency (the old behavior).
ping -v	Verbose command output.
ping -V	Shows version and exits.
ping -w [deadline]	Specifies a deadline (or timeout), in seconds, before ping exits regardless of how many packets have been sent or received.
ping -W [timeout]	Time to wait for a response, in seconds. The option affects only timeout in absense of any responses, otherwise ping waits for two RTTs.

Route

Route enables the manipulation and viewing of a computer’s routing table. In other words, it lets you add or remove entries in the routing table.
In Windows the route command follows this syntax:
route [-f] [-p] [-4 or -6][Command [Destination] [mask Netmask] [Gateway] [metric Metric]] [if Interface]]

Route command switches (Windows)	Description
route -f	Clears the routing tables of all gateway entries. If this is used in conjunction with one of the commands, the tables are cleared prior to running the command.
route -p	When used with the ADD command, makes a route persistent across boots of the system. By default, routes are not preserved when the system is restarted.
route -4	Force using IPv4.
route -6	Force using IPv6.
route ADD	Adds a route.
route CHANGE	Modifies an existing route.
route DELETE	Deletes a route.
route PRINT	Prints a route

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A route command example in Windows would be:
route add 10.41.0.0 mask 255.255.0.0 10.27.0.1 (adds a persistent route to the destination 10.41.0.0 with the subnet mask of 255.255.0.0 and the next hop address of 10.27.0.1).
The standard route command syntax in Linux is as follows:
route [options] [add or del] [-net or -host] address [modifiers]

Route command switches (Linux)	Description
route	Displays the host’s routing tables.
route -A [family] [add] or route –[family] [add]	Uses the specified address family with add or del. Valid families are inet (DARPA Internet), inet6 (IPv6), ax25 (AMPR AX.25), netrom (AMPR NET/ROM), ipx (Novell IPX), ddp (Appletalk DDP), and x25 (CCITT X.25).
route -C or –cache	Operates on the kernel’s routing cache instead of the forwarding information base (FIB) routing table.
route -e or -ee	Uses the netstat-r format to display the routing table. -ee will generate a very long line with all parameters from the routing table.
route -F or –fib	Operates on the kernel’s Forwarding Information Base (FIB) routing table (default behavior).
route -h or –help	Prints the help message.
route -n	Shows numerical IP addresses and bypass host name resolution.
route -v or –verbose	Enables verbose command output.
route -V or –version	Dispays the version of net-tools and the route command.
route add or del	Adds or delete a route in the routing table.
route [add or del] dev [interface]	Associates a route with a specific device. If dev [interface] is the last option on the command line, the word dev may be omitted.
route [add or del] [default] gw [gw]	Routes packets through the specified gateway.
route [add or del] -host	Specifies that the target is a host (not a network).
route [add or del] -irtt [I]	Sets the initial round trip time (IRTT) for TCP connections over this route to [I] milliseconds (1-12000). This is typically only used on AX.25 networks. If omitted the RFC 1122 default of 300ms is used.
route [add or del] -net	Specifies that the target is a network (not a host).
route [add or del] [-host or -net] netmask [mask]	Sets the subnet [mask].
route [add or del] metric [n]	Sets the metric field in the routing table (used by routing daemons) to the value of [n].
route [add or del] mod, dyn, or reinstate	Install a dynamic or modified route. These flags are for diagnostic purposes, and are generally only set by routing daemons.
route [add or del] mss [bytes]	Sets the TCP Maximum Segment Size (MSS) for connections over this route to the number of [bytes] specified.
route [add or del] reject	Installs a blocking route, which will force a route lookup to fail. This is used to mask out networks before using the default route. This is not intended to provide firewall functionality.
route [add or del] window [W]	Set the TCP window size for connections over this route to the value of [W] bytes. This is typically only used on AX.25 networks and with drivers unable to handle back-to-back frames.

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These examples should clarify the syntax of the route command.
route add -net 127.0.0.0
Adds the normal loopback entry, using netmask 255.0.0.0 (class A net, determined from the destination address) and associated with the “lo” device (assuming this device was previously set up correctly with ifconfig).
route add -net 192.56.76.0 netmask 255.255.255.0 dev eth0
Adds a route to the network 192.56.76.x via “eth0″. The Class C netmask modifier is not really necessary here because 192.* is a Class C IP address. The word “dev” can be omitted here.
route add default gw 192.168.100.1 dev eth0
Adds a default route (which will be used if no other route matches). All packets using this route will be gatewayed through 192.168.100.1 for interface eth0. The word “dev” can be omitted here.

Telnet

Telnet is a network protocol that allows for connections and user sessions on remote hosts. It is called a ‘terminal emulator’ because it lets you work on a remote host as if you were seated in front of it and using its monitor, keyboard, and mouse. A computer terminal is defined as “an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system.” The input and output devices described above create a terminal environment, and telnet lets you mimic that environment on another computer (also referred to as “getting a shell” on the remote host).
Any information security professional worth his salt will tell you that using telnet is not recommended as all logins, passwords and commands are transferred in clear text. An attacker may eavesdrop on a telnet session and obtain the credentials and sensitive data of other users. The Secure Shell (SSH) protocol has effectively replaced telnet; however, in closed, restricted networks where there is zero chance of an attacker performing packet sniffing, telnet can be used. You should still make efforts to phase telnet out and replace it with SSH. SSH is such an important and useful protocol that it will be covered in a forthcoming article on this website. Until then you can learn more about SSH and its uses here.
This page supposedly lists free telnet servers that you can connect to and practice on.
By running a telnet connection to the open ports on a host, you can view the banners that reveal which service is answering on those specific ports. Many email, FTP, and web servers will respond to telnet connection requests with the name and version of their email, FTP, and web software (such as Exchange, IIS, etc). If you want to grab the banner of a web server, type:
telnet 80, followed by: HEAD/HTTP/1.0.
Telnet is not installed by default on Windows Vista, 7, or 2008. To learn how to get telnet running on these operating systems, use the instructions located on TechNet.
The standard telnet command syntax in Windows is:
telnet [-a][-eescape_char][-flog_file][-luser_name][-tterm]host [port]
(where host specifies the NetBIOS name, fully qualified domain name, or IP address of the host with which you want to create a telnet connection, and port specifies the TCP port on which you want to create a telnet connection. The default telnet port is 23).

Telnet command switches (Windows)	Description
telnet [servername] [portnumber]	Starts the telnet client and attempts to initiate a session with [servername] on [portnumber]. If no [portnumber] is provided, the default port 23 is used.
telnet -a	Instructs telnet.exe to log on to the host using the credentials of the user who is currently logged on to the client.
telnet -e [escape_char]	Specifies an escape character, which displays the telnet command prompt. The default escape character is Ctrl+].
telnet -f [log_file]	Creates a client-side log file and turns on client-side logging for the current session. The [log_file] parameter must consist of a path and file name.
telnet -l [user_name]	Instructs telnet.exe to log on to the host using the user account that is specified in [user_name]. The user account specified in [user_name] must have Telnet logon rights on the host.
telnet -t [term]	Specifies the terminal type. The default terminal type is ANSI. Other valid terminal types include VT52, VT100, and VTNT.
close or c	Closes an existing Telnet connection.
display	Use the display command to view the current settings for the telnet client. The display command lists the current operating parameters. If you are in a telnet session (connected to a telnet server), to modify the parameters, press CTRL+]. This escapes from the telnet session. (To return to the Telnet session, press ENTER.)
display WILL AUTH	Enables NTLM authentication.
display WONT AUTH	Indicates that NTLM authentication is disabled, and that user IDs and passwords will be sent in plaintext.
display WILL TERM TYPE
display WONT TERM TYPE
display LOCALECHO off
display LOCALECHO off
enter	Go to the connected session (if it exists).
open or o	Establish a telnet connection with a host computer or remote server. For example, o telnet.org 44 will attempt to connect your computer to telnet.org using port 44.
quit or q	Exit telnet.
send	Sends commands to the telnet server.
send ao	Abort output command.
send ayt	“Are you there?” command.
send brk	Sends a break signal.
send esc	Sends the current escape character.
send ip	Interrupts the current process.
send synch	Performs the telnet synch operation.
set	Set the terminal type for the connection, turn on local echo, set authentication to NTLM, set the escape character, and set up logging.
set bsasdel	Backspace will be sent as delete.
set crlf	Sets new-line mode; causes the RETURN key to send the combination of carriage return and line feed: 0x0D, 0x0A. When turned off, it sends only the carriage return character, 0x0D.
set delasbs	Delete will be sent as backspace.
set escape [character]	Specifies the [character] that will switch from telnet session mode to the telnet command mode. While in telnet command mode, press ENTER to return to telnet session mode.
set localecho	Turns on local echo. All characters typed will be displayed on the local console. Remote Telnet servers usually send each character typed back, so setting this option could result in duplicate characters displayed for each one typed.
set logfile [name]	Specifies the [name] of the file to which the telnet log is written for this session. If you do not specify the path of the file, it is created in your current directory. Specifying a log file also turns on logging. The log file must be on your local computer.
set logging	Turns on logging for this session. If no log file is set, an error message is displayed.
set mode {console | stream}	Changes the mode of operation from console mode to stream mode. Setting the mode to stream turns off the ANSI or other escape commands that attempt to control cursor placement or clearing the screen. Setting the mode to console causes Telnet server to include ANSI or other escape commands to control cursor positioning within a fixed window of x rows and y columns. Stream mode is useful when you do not want the Telnet client to emulate a terminal. The output is sent as if there is no terminal window with x rows and y columns. Some applications that use telnet for file transfer fail to work correctly when console mode is enabled.
set ntlm	Turns on NTLM authentication.
set term {ansi | vt100 | vt52 | vtnt}	Specifies the type of terminal you want telnet client to emulate. You should use the VT100 terminal type if you are running normal command-line applications.
status	Determines whether the computer running the telnet client is connected.
unset	Turns off an option previously set by using the set command

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The standard telnet command syntax in Linux is:
telnet [-l user] [-a] host-name [port]
telnet [-8EFKLacdfrx] [-X authtype] [-b hostalias] [-e escapechar] [-k realm] [-l user] [-n tracefile] [host [port]]

Telnet command switches (Linux)	Description
telnet	Enters telnet command mode (prompt becomes telnet>).
telnet -a	Attemps automatic login to the remote host.
telnet -b [hostalias]	Uses bind to bind the local socket to an aliased address (see ifconfig –alias) or to the address of an interface different from the one selected by connect.
telnet -c	Disables the reading of the user’s .telnetrc file. The .telnetrc file contains the setup information for a telnet session. It is a hidden file in your home directory and must be readable by the user logging in.
telnet -d	Enables debugging at the socket level.
telnet -e [escape_char]	Sets the initial telnet escape character to [escapechar]. If [escapechar] is omitted, then there will be no escape character.
telnet -E	Stops any character from being recognized as an escape character.
telnet -f	If Kerberos V5 authentication is being used, allows the local credentials to be forwarded to the remote host.
telnet -F	If Kerberos V5 authentication is being used, allows the local credentials to be forwarded to the remote host, including any credentials that have already been forwarded into the local environment.
telnet -k [realm]	If Kerberos authentication is being used, requests that telnet obtain tickets for the remote host in realm [realm] instead of the remote host’s realm.
telnet -K	Prevents automatic login to the remote host.
telnet -l [user]	When connecting to the remote host, if the remote host understands the ENVIRON option, then [user] will be sent as the value for the variable USER. This option implies the -a option and may also be used with the open command.
telnet -L	Specifies an 8-bit data path on output. This causes the BINARY option to be negotiated on output.
telnet -n [trace_file]	Opens [trace_file] for recording the trace information.
telnet -r	Specifies a user interface similar to rlogin. In this mode, the escape character is set to the tilde (~) character, unless modified by the -e option.
telnet -x	Enables encryption of the data stream, if possible.
telnet -X [auth_type]	Disables the [auth_type] type of authentication.
telnet> close	Ends the session and closes the current connection.
telnet> display	Displays the telnet operating parameters.
telnet> environ	Changes the environment variables. Possibilities include: . define: defines an environment variable. export: marks an environment variable for automatic export. list: lists the current environment variables. send: sends an environment variable. undefine: undefines an environment variable. unexport: don’t mark an environment variable for automatic export. ?: prints help information.
telnet> logout	Forcibly logs out the remote user and closes the connection.
telnet> mode [mode]	Tries to enter line or character mode where [mode] is one of the following: . character: disables LINEMODE option (or disables obsolete line-by-line mode). line: enables LINEMODE option (or enables obsolete line-by-line mode). These two require the LINEMODE option to be enabled. isig: enables signal trapping. -isig: disables signal trapping. edit: enables character editing. -edit: disables character editing. softtabs: enables tab expansion. -softtabs: disables character editing litecho: enables literal character echo. -litecho: disables literal character echo. ?: prints help information.
telnet> open [port]	Connects to a host.
telnet> quit	Exits telnet.
telnet> send	Transmits special characters as defined below. . abort: sends Telnet ‘Abort Process’. ao: sends Telnet Abort output. ayt: sends Telnet ‘Are You There’. brk: sends Telnet Break. ec: sends Telnet Erase Character. el: sends Telnet Erase Line. eof: sends Telnet End of File Character. eor: sends Telnet ‘End of Record’. escape: sends current escape character. ga: sends Telnet ‘Go Ahead’ sequence. getstatus: sends request for STATUS. ip: sends Telnet Interrupt Process. nop: sends Telnet ‘No operation’. susp: sends Telnet ‘Suspend Process’. synch: performs Telnet ‘Synch operation’. ?: displays send options.
telnet> set	Sets operating parameters as described below. . debug: enables debugging. echo: character to toggle local echoing on/off. escape: character to escape back to telnet command mode. netdata: enables printing of hexadecimal network data (debugging). options: enables viewing of options processing (debugging). prettydump: enables output of “netdata” to user readable format (debugging). rlogin: rlogin escape character. termdata: enables (debugging) toggle printing of hexadecimal terminal data. tracefile: the file to write trace information to. ?: display help information. . The following need ‘localchars’ to be toggled ‘true’: . eof: character to cause an EOF. flushoutput: character to cause an Abort Output. interrupt: character to cause an Interrupt Process. quit: character to cause an Abort process. . The following are for local editing in linemode: . autoflush: enables flushing of output when sending interrupt characters. autosynch: enables automatic sending of interrupt characters in urgent mode. crlf: enables sending carriage returns as telnet crmod: enables mapping of received carriage returns. erase: character to use to erase a character. forw1: alternate end of line character. forw2: alternate end of line character. inbinary: enables receiving of binary data. kill: character to use to erase a line. lnext: character to use for literal next. localchars: enables local recognition of certain control characters. outbinary: enables sending of binary data. reprint: character to use for line reprint. start: character to use for XON. stop: character to use for XOFF. skiprc: enable don’t read ~/.telnetrc file. susp: character to cause a Suspend Process. worderase: character to use to erase a word.
telnet> slc	Changes the state of special charaters as follows: . check: Verify remote special character definitions. export: Use local special character definitions. import: Use remote special character definitions. ?: Print help information.
telnet> status	Prints connection status information.
telnet> toggle	Toggles operating parameters as follows: . autoflush: toggle flushing of output when sending interrupt characters. autosynch: toggle automatic sending of interrupt characters in urgent mode. binary: toggle sending and receiving of binary data. crlf: toggle sending carriage returns as telnet . crmod: toggle mapping of received carriage returns. debug: toggle debugging. inbinary: toggle receiving of binary data. localchars: toggle local recognition of certain control characters. netdata: toggle printing of hexadecimal network data (debugging). options: toggle viewing of options processing (debugging). outbinary: toggle sending of binary data. prettydump: toggle output of “netdata” to user readable format (debugging). skiprc: toggle don’t read ~/.telnetrc file. termdata: toggle (debugging) toggle printing of hexadecimal terminal data.
telnet> unset	Unsets operating parameters; the opposite of telnet> set.
telnet> z	Suspends a telnet session.
telnet> !	Invokes a subshell.
telnet> ?	Prints the help message.

Traceroute and Tracert

Traceroute (tracert in Windows) is a network diagnostic tool for displaying the route (path) of packets and measuring their transit delays across an IP network. Traceroute works by increasing the TTL value of each successive set of packets sent. The first set of packets has a hop limit value of 1, so they are not forwarded by the first router. The next set has a hop limit value of 2, so that the second router will send the error reply. This continues until the destination host receives the packets and returns an ICMP Echo Reply message. In other words, the TTL value on each packet increments by one after each hop is reached. Then the packet returns, ensuring that the response comes back explicitly from that hop (its name and IP address are also displayed). Traceroute uses the returned ICMP messages to produce a list of routers that the packets have traversed. The timestamp values returned for each router along the path are the delay (latency) values measured in milliseconds for each packet.
The traceroute/tracert tool will time out (indicated in the command output by an asterisk) when it encounters a firewall or a packet-filtering router.
Traceroute in Linux and Unix uses UDP by default, while tracert in Windows uses ICMP.
The standard tracert command syntax is:
tracert [-d] [-h maximum_hops] [-j host-list] [-w timeout] [-R] [-S srcaddr] [-4] [-6] target_name

Tracert command switches	Description
tracert [destination]	Traces the path to the host or IP address named [destination].
tracert -d	Prevents tracert from attempting to resolve the IP addresses of intermediate routers to their names (which can speed up the display of tracert results).
tracert -h [MaxHops]	Specifies the maximum number of hops in the path to search for the target (destination). The default is 30 hops.
tracert -j [HostList]	Specifies that Echo Request messages use the Loose Source Route option in the IP header with the set of intermediate destinations specified in [HostList]. With loose source routing, successive intermediate destinations can be separated by one or multiple routers. The maximum number of addresses or names in the host list is 9. The [HostList] is a series of IP addresses (in dotted decimal notation) separated by spaces.
tracert -w [timeout]	Specifies the amount of time in milliseconds to wait for the ICMP Time Exceeded or Echo Reply message corresponding to a given Echo Request message to be received. If not received within the time-out, an asterisk (*) is displayed. The default time-out is 4000 (4 seconds).
tracert -R	Trace round-trip path (IPv6-only).
tracert -S [srcaddr]	Source address to use (IPv6-only).
tracert -4	Force using IPv4.
tracert -6	Force using IPv6.

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Traceroute command switches	Description
traceroute -A	Performs autonomous system (AS) path lookups in routing registries and prints results directly after the corresponding addresses.
traceroute –back	Prints the number of backward hops when it seems different than the forward direction. This number is guessed in the assumption that remote hops send reply packets with the initial TTL set to either 64, 128, or 255 (which seems a common practice). It is printed as a negative value in a form of ‘-NUM’ .
traceroute -d	Enables socket level debugging (when the Linux kernel supports it).
traceroute -e	Shows the ICMP extensions (RFC 4884). The general form is CLASS/TYPE, followed by a hexadecimal dump.
traceroute -f [N]	Sets the initial TTL to [N] hops.
traceroute -F	Sets the ‘don’t fragment’ bit.
traceroute –fwmark=[mark]	Sets the firewall [mark] for outgoing packets (since Linux kernel version 2.6.25). Firewall marks are described here.
traceroute -g [gw_addr]	Tells traceroute to add an IP source routing option to the outgoing packet that tells the network to route the packet through the specified [gw_addr] (most routers disable source routing for security reasons). In general, several gateways are allowed (comma separated).
traceroute –help	Prints the help message.
traceroute -i [interface]	Specifies the [interface] through which traceroute should send packets. By default, the interface is selected according to the routing table.
traceroute -I [raw or dgram]	Uses ICMP ECHO for probes insteaf of UDP. Options are [raw] sockets or [dgram] ICMP sockets.
traceroute -m [max_ttl]	Specifies the maximum number of hops (i.e., the max TTL value). The default is 30.
traceroute -M [method]	Uses the specified [method] for traceroute operations. Default traditional UDP method has the name default; ICMP (-I) and TCP (-T) have names icmp and tcp, respectively. Method-specific options can be passed by -O.
traceroute –mtu	Discovers the maximum transmission unit (MTU) along the path being traced. Implies -F -N 1.
traceroute -n	Shows numerical IP addresses and bypasses attempts at host name resolution.
traceroute -N [num_probes]	Specifies the number of probe packets sent out simultaneously. Sending several probes concurrently can speed up traceroute. The default value is 16. Note that if you use ICMP probes, some routers and hosts can use ICMP rate throttling.
traceroute -O [option]	Specifies some method-specific [option]. Several options are separated by commas (or they use several -O on the command line).
traceroute -p [port]	Sets the destination base UDP port number to [port]; default is 33434. For ICMP tracing, specifies the initial ICMP sequence value (incremented by each probe). For TCP, specifies just the (constant) destination port to connect to.
traceroute -P [proto]	Uses the raw packet of the specified protocol for tracerouting. The default protocol is 253 (RFC 3692).
traceroute -q [num_queries]	Sets the number of probe packets per hop. The default is 3.
traceroute -r	Bypasses the normal routing tables and sends directly to a host on an attached network. If the host is not on a directly attached network, an error is returned. This option can be used to ping a local host through an interface that has no route through it.
traceroute raw -P [proto]	Sends raw packets of protocol [proto] (no protocol-specific headers are used, just the IP header only). Implies -N 1.
traceroute -s [src_addr]	Specifies an alternative source address for the outgoing traceroute packets.
traceroute –sport=[port]	Chooses the source port to use. Implies -N 1.
traceroute -t [ToS]	Set the Type of Service [ToS] and Precedence value (0-255 decimal). Useful values are 16 (low delay) and 8 (high throughput). For IPv6, -t sets the Traffic Control value.
traceroute -T [opt]	Uses the TCP protocol for traceroute probes. Available options are: syn, ack, fin, rst, psh, urg, ece, and cwr. If these options don’t make sense to you, you should read my article on Nmap.
traceroute -T ecn	Sends SYN packets with TCP flags ECE and CWR (for Explicit Congestion Notification, RFC 3168).
traceroute -T flags=[num]	Sets the flags field in the TCP header to [num].
traceroute -T info	Prints the TCP flags of the final TCP replies when the target host is reached. Determines whether an application is listening the port.
traceroute -T mss=[num]	Uses the value of [num] for maximum segment TCP header option (when SYN).
traceroute -T sack, timestamps, window_scaling	Uses the corresponding TCP header option in the outgoing probe packet.
traceroute -T sysctl	Uses current sysctl (/proc/sys/net/*) setting for the TCP header options above and ECN. Always set by default, if nothing else is specified.
traceroute tcpconn	An initial implementation of the TCP method simply using the connect call, which does a full TCP session opening. Not recommended for normal use because a destination application is always affected.
traceroute -U	Uses UDP datagrams with a constant destination port (default 53, DNS). Like the -T (TCP) option, this can be intended to bypass firewall as well.
traceroute -UL coverage=[num]	Uses UDP Lite datagrams for probes (with a constant destination port, 53) where [num] determines the checksum coverage sent. See here for reference.
traceroute -V	Prints the version of traceroute.
traceroute -w [wait_time]	Sets the time (in seconds) to wait for a response to a probe (the default is 5 seconds).
traceroute -z [sendwait]	Sets the minimal time interval between probes (default is 0). If the value is more than 10, then it specifies a number in milliseconds, otherwise it is a number of seconds. Useful when some routers use rate-limiting for ICMP messages.
traceroute -4 or -6	Explicitly forced IPv4 or IPv6. By default, the program will try to resolve the name given, and choose the appropriate protocol automatically. If resolving a host name returns both IPv4 and IPv6 addresses, traceroute will use IPv4.

Whois

whois is a domain name query tool. Some of the information it provides on domain names includes: registered address, technical and DNS contacts, contact email addresses, contact phone number, and the expiration date of the domain name registration.
Whois commands are commonly executed from network query-oriented websites, as described here. However, you can run whois from a command line interface as well. Whois is easily available in Linux and Unix (may be available as jwhois), but must be installed manually on Windows. Note that the version of whois published by Marco d’Itri of linux.it uses very different options from the standard whois/jwhois.

Whois command switches (Windows)	Description
whois [domainname] [whois.server]	Performs a whois query on [domainname] using the whois server specified.

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Whois command switches (Linux)	Description
whois -a or –raw	Disables reformatting of the query.
whois -c [file] or –config=[file]	Uses [file] as a configuration file instead of jwhois.conf.
whois -d or –disable-cache	Disables reading from (and writing to) the cache.
whois -f or –force-lookup	Forces a query to be made to a host even if a current object is available from the cache.
whois -h [host] or –host=[host]	Overrides any hosts in the configuration file and queries [host] directly.
whois –help	Prints the help message.
whois -i or –display-redirections	Displays every step in a redirection (the default is to display only the last answer).
whois -n or -no-redirect	Disables redirection from one server to another.
whois -p [port] or –port=[port] [host]	Specifies a port number to use when querying a host. For example, whois –port=9103 10.20.30.31 forces a test on this host’s port 9103
whois -r or –rwhois	Forces the query to use the rwhois protocoll instead of HTTP or whois.
whois –rwhois-display=[display]	Asks receiving rwhois servers to display the results in the specified [display] instead of the default dump display.
whois –rwhois-limit=[limit]	Asks receiving rwhois servers to limit their responses to [limit] matches.
whois -s or –no-whoisservers	Disables the built-in support for whois-servers.net.
whois -v or –verbose	Outputs verbose debugging information while running. You can increase the verbosity by giving several verbose commands to jwhois, such as -vv.
whois –version	Displays the version, authors and licensing information of whois.