Exercise 24. Networking: interface configuration, ifconfig, netstat, iproute2, ss, route
This exercise is a big one in terms of info dumped on you, and if you are not familiar with networking this is gonna hurt. If you feel absolutely lost, skip to the Do This part right away and be done with it. To understand this part properly you should be at least superfluously familiar with the following basic concepts of networking:
- Communications protocol — a communications protocol which is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications.
- Ethernet — family of computer networking technologies for local area networks (LANs).
- MAC address — an unique identifier assigned to network interfaces for communications on the physical network segment. Example: 08:00:27:d4:45:68.
- TCP/IP — the Internet protocol suite is the set of communications protocols used for the Internet and similar networks, and generally the most popular protocol stack for wide area networks. It is commonly known as TCP/IP, because of its most important protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP)
- IP — the Internet Protocol (IP) is the principal communications protocol used for relaying datagrams (also known as network packets) across an internetwork using the Internet Protocol Suite.
- IP address — an Internet Protocol address. Example: 10.0.2.15
- Port — application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. Example: 22
- Network socket — an endpoint of an inter-process communication flow across a computer network. Today, most communication between computers is based on the Internet Protocol; therefore most network sockets are Internet sockets.
- Local socket address — local IP address and port number, example: 10.0.2.15:22.
- Remote socket address — remote IP address and port number, only for established TCP sockets. Example: 10.0.2.2:52173.
- Socket pair — communicating local and remote sockets, only TCP protocol. Example: (10.0.2.15:22, 10.0.2.2:52173).
- Subnet mask — logically visible subdivision of an IP network. Example: /24 or, in another notation, 255.255.255.0.
- Routing — the process of selecting paths in a network along which to send network traffic.
- Default gateway — in computer networking, a gateway is a node (a router) on a TCP/IP network that serves as an access point to another network. A default gateway is the node on the computer network that the network software uses when an IP address does not match any other routes in the routing table. Example: 10.0.2.2.
- Broadcast address — logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. A message sent to a broadcast address is typically received by all network-attached hosts, rather than by a specific host. Example: 10.0.2.255.
- ICMP — Internet Control Message Protocol, example usage: ping 10.0.2.2.
- TCP — Transmission Control Protocol. Establishes connection before data can be exchanged, therefore reliable by design. Example users: SSH, HTTP.
- UDP — User Datagram Protocol. Transfers data without establishing connection, therefore unreliable by design. Example users: DNS.
If some of this concepts are not familiar to you, no worries.
- Read corresponding Wikipedia articles until you achieve at least superfluous understanding (but good old hardcore grinding is of course better).
- Watch this videos from http://www.visualland.net.cn/:
- Expand IP Address tree node in the left of the site and work you way through it.
- Expand TCP tree node and do the same.
- Read Linux networking concepts introduction. This guide is good because it even acknowledges that The Internet is for porn.
Let us continue. This is list of Linux network-related commands:
- ifconfig — configure and view status of network interface.
- netstat — print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
- ip — show / manipulate routing, devices, policy routing and tunnels.
- ss — another utility to investigate sockets.
Now let us examine what information each command can show us. We will start with ifconfig.
user1@vm1:~$ sudo ifconfig eth0 Link encap:Ethernet HWaddr 08:00:27:d4:45:68 # (1), (2), (3) inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0 # (4), (5), (6), (7) inet6 addr: fe80::a00:27ff:fed4:4568/64 Scope:Link # (8), (9), (10) UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 # (11), (12), (13), (14), (15), (16) RX packets:35033 errors:0 dropped:0 overruns:0 frame:0 # (17), (18), (19), (20), (21), (22) TX packets:28590 errors:0 dropped:0 overruns:0 carrier:0 # (23), (24), (25), (26), (27), (28) collisions:0 txqueuelen:1000 # (29), (30) RX bytes:6360747 (6.0 MiB) TX bytes:21721365 (20.7 MiB) # (31), (32) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:8 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:560 (560.0 B) TX bytes:560 (560.0 B)
We can see that there are two network interfaces in vm1, eth0 and lo. lo is a loopback interface, which is used to connect to client-server programs on the same machine.
Let us see what information we have on eth0, which is a VirtualBox pseudo network interface:
| Field | Description | Field | Description |
|---|---|---|---|
| (1) Link | Physical options | (17) RX | Receive (abbreviation) |
| (2) encap | Encapsulation type | (18) packets | Total number of packets |
| (3) Hwaddr | MAC-address | (19) errors | Total number of packets with errors |
| (4) inet | Address family (IPv4) | (20) dropped | Dropped packets (low system memory?) |
| (5) addr | IPv4 address | (21) overruns | Packets come faster than they can be processed |
| (6) Bcast | Broadcast address | (22) frame | Invalid frames received |
| (7) Mask | Network mask | (23) TX | Transmit (abbreviation) |
| (8) inet6 | Address family (IPv6) | (24) packets | Total number of packets |
| (9) addr | IPv6 address | (25) errors | Total number of packets with errors |
| (10) Scope | Address scope (host, link, global) | (26) dropped | Dropped packets (low system memory?) |
| (11) UP | Interface is functioning | (27) overruns | An app sends packes very fast? (I am not sure) |
| (12) BROADCAST | It can send traffic to all hosts at once | (28) carrier | Loss of link carrier wave |
| (13) RUNNING | It is ready to accept data (I am not sure) | (29) collisions | Packed collision did occur |
| (14) MULTICAST | It can send and receive multicast packets | (30) txqueuelen | Transmin queue length for outgoing packets |
| (15) MTU | Its Maximum transfer unit | (31) RX bytes | Total received bytes |
| (16) Metric | Route cost (not used in Linux) | (32) TX bytes | Total sent bytes |
That sure is a lot. But then again, for now only important fields are:
- (5) addr — IPv4 address.
- (6) Bcast — Broadcast address.
- (7) Mask — Network mask.
- (11) UP — Interface is functioning.
- (13) RUNNING — It is ready to accept data.
- (19) errors and (25) errors — if there is something different from zero here, we have problems.
Now let us see what netstat can show us.
user1@vm1:~$ sudo netstat -ap Active Internet connections (servers and established) #(1) (2) (3) (4) (5) (6) (7) Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 *:sunrpc *:* LISTEN 580/portmap tcp 0 0 *:ssh *:* LISTEN 900/sshd tcp 0 0 localhost:smtp *:* LISTEN 1111/exim4 tcp 0 0 *:36286 *:* LISTEN 610/rpc.statd tcp 0 0 10.0.2.15:ssh 10.0.2.2:52191 ESTABLISHED 12023/sshd: user1 [ tcp 0 0 10.0.2.15:ssh 10.0.2.2:48663 ESTABLISHED 11792/sshd: user1 [ tcp6 0 0 [::]:ssh [::]:* LISTEN 900/sshd tcp6 0 0 ip6-localhost:smtp [::]:* LISTEN 1111/exim4 udp 0 0 *:bootpc *:* 843/dhclient udp 0 0 *:sunrpc *:* 580/portmap udp 0 0 *:52104 *:* 610/rpc.statd udp 0 0 *:786 *:* 610/rpc.statd #(8) (9) (10) (11) (12) (13) (14) (15) Active UNIX domain sockets (servers and established) Proto RefCnt Flags Type State I-Node PID/Program name Path unix 2 [ ACC ] STREAM LISTENING 3452 786/acpid /var/run/acpid.socket unix 6 [ ] DGRAM 3407 751/rsyslogd /dev/log unix 2 [ ] DGRAM 1940 214/udevd @/org/kernel/udev/udevd unix 2 [ ] DGRAM 88528 30939/sudo unix 3 [ ] STREAM CONNECTED 68565 12023/sshd: user1 [ unix 3 [ ] STREAM CONNECTED 68564 12026/1 unix 2 [ ] DGRAM 68563 12023/sshd: user1 [ unix 3 [ ] STREAM CONNECTED 66682 11792/sshd: user1 [ unix 3 [ ] STREAM CONNECTED 66681 11794/0 unix 2 [ ] DGRAM 66680 11792/sshd: user1 [ unix 2 [ ] DGRAM 3465 843/dhclient unix 2 [ ] DGRAM 3448 786/acpid unix 3 [ ] DGRAM 1945 214/udevd unix 3 [ ] DGRAM 1944 214/udevd
I used two parameters to modify netstat output. -a parameter told netstat to show us all connections, both established, like your current ssh session through which your are typing, and listening, like sshd daemon waiting for new connections. -p told netstat to show which program owns each connection.
| Active Internet Connections (servers and established) | |
|---|---|
| Field | Description |
| (1) Proto | The protocol (tcp, udp, raw) used by the socket. |
| (2) Recv-Q | The count of bytes not copied by the user program connected to this socket. |
| (3) Send-Q | The count of bytes not acknowledged by the remote host. |
| (4) Local Address | Address and port number of the local end of the socket. |
| (5) Foreign Address | Address and port number of the remote end of the socket. |
| (6) State | ESTABLISHED, SYN_SENT, SYN_RECV, FIN_WAIT1, FIN_WAIT2, TIME_WAIT, CLOSE, CLOSE_WAIT, LAST_ACK, LISTEN, CLOSING, UNKNOWN |
| (7) PID | Slash-separated pair of the process id (PID) and process name of the process that owns the socket. |
| Active UNIX domain sockets (servers and established) | |
| Field | Description |
| (8) Proto | The protocol (usually unix) used by the socket. |
| (9) RefCnt | The reference count (i.e. attached processes via this socket). |
| (10) Flags | The flags displayed is SO_ACCEPTON (displayed as ACC), SO_WAITDATA (W) or SO_NOSPACE (N). |
| (11) Type | SOCK_DGRAM, SOCK_STREAM, SOCK_RAW, SOCK_RDM, SOCK_SEQPACKET, SOCK_PACKET, UNKNOWN. |
| (12) State | FREE, LISTENING, CONNECTING, CONNECTED, DISCONNECTING, (empty), UNKNOWN. |
| (13) I-Node | I-Node of socket file. |
| (14) PID | Process ID (PID) and process name of the process that has the socket open. |
| (15) Path | This is the path name as which the corresponding processes attached to the socket. |
Not all fields are actually important. Usually you have to look only in Active Internet Connections (servers and established) section, and use this fields:
- (1) Proto — The protocol (tcp, udp, raw) used by the socket.
- (4) Local Address — address and port number of the local end of the socket.
- (5) Foreign Address — address and port number of the remote end of the socket, only for socket pairs.
- (6) State — for now you should know only two states: LISTEN and ESTABLISHED. First means that you may connect to this socket, second means that this socket already is connected, and in this cases netstat shows you socket pair.
ip is a program similar to ifconfig with extended capabilities. It is from iproute2 suite which is intended to replace ifconfig some day. Example output:
user1@vm1:~$ sudo ip addr show 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo inet6 ::1/128 scope host valid_lft forever preferred_lft forever # (1) (2) (3) (4) (5) (6) (8) (9) 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:d4:45:68 brd ff:ff:ff:ff:ff:ff # (9), (10), (11) inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0 # (12), (13), (14) inet6 fe80::a00:27ff:fed4:4568/64 scope link # (15), (16) valid_lft forever preferred_lft forever # (17), (18), (19)
Again, let us see what information we have on eth0:
| Field | Description |
|---|---|
| (1) BROADCAST | It can send traffic to all hosts at once |
| (2) MULTICAST | It can send and receive multicast packets |
| (3) UP | It is functioning, logical state |
| (4) LOWER_UP | Driver signals L1 up (since Linux 2.6.17) |
| (5) MTU | Maximum transfer unit |
| (6) qdisc | Queueing discipline, basically traffic scheduler policy |
| (8) State | Physical state (carrier sense?) |
| (9) qlen | Transmin queue length for outgoing packets |
| (10) link | Physical options |
| (11) ether | Encapsulation type, MAC-address |
| (12) brd | Data Link Layer (physical) broadcast address |
| (13) inet | IPv4 address family address |
| (14) brd | IPv4 boradcast |
| (15) scope | IPv4 Address scope (host, link, global) |
| (16) inet6 | IPv6 address family address |
| (17) scope | IPv6 Address scope (host, link, global |
| (18) valid_lft | IPv6 source address selection policy |
| (19) preffered_lft | IPv6 source address selection policy |
You already know which parameters are important (the same as in ifconfig).
ss is basically contemporary netstat with extended capabilities. This is its example output, interpretation of which is left as exercise for you:
user1@vm1:~$ sudo ss -ap | cut -c1-200 State Recv-Q Send-Q Local Address:Port Peer Address:Port LISTEN 0 128 *:sunrpc *:* users:(("portmap",580,5)) LISTEN 0 128 :::ssh :::* users:(("sshd",900,4)) LISTEN 0 128 *:ssh *:* users:(("sshd",900,3)) LISTEN 0 20 ::1:smtp :::* users:(("exim4",1111,4)) LISTEN 0 20 127.0.0.1:smtp *:* users:(("exim4",1111,3)) LISTEN 0 128 *:36286 *:* users:(("rpc.statd",610,7)) ESTAB 0 0 10.0.2.15:ssh 10.0.2.2:52191 users:(("sshd",12023,3),("sshd",12026,3)) ESTAB 0 0 10.0.2.15:ssh 10.0.2.2:48663 users:(("sshd",11792,3),("sshd",11794,3))
This is for working with interface, connetctions and interface addresses. But what about network routes? You can get information about them using several commands as well:
user1@vm1:~$ sudo route -n Kernel IP routing table # (1) (2) (3) (4) (5) (6) (7) (8) Destination Gateway Genmask Flags Metric Ref Use Iface 10.0.2.0 * 255.255.255.0 U 0 0 0 eth0 default 10.0.2.2 0.0.0.0 UG 0 0 0 eth0 user1@vm1:~$ sudo netstat -nr Kernel IP routing table # (9) Destination Gateway Genmask Flags MSS Window irtt Iface 10.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 0.0.0.0 10.0.2.2 0.0.0.0 UG 0 0 0 eth0 user1@vm1:~$ sudo ip route show # (10) (11) (12) (13) (14) 10.0.2.0/24 dev eth0 proto kernel scope link src 10.0.2.15 #(15) (16) default via 10.0.2.2 dev eth0
Let us work through fields onece more:
| Field | Description |
|---|---|
| (1) Destination | The destination network or destination host. |
| (2) Gateway | The gateway address or '*' if none set. |
| (3) Genmask | The netmask for the destination net; '255.255.255.255' for a host destination and '0.0.0.0' for the default route. |
| (4) Flags | Up, Host, Gateway, Reinstate, Dynamically installed, Modified, Addrconf, Cache entry, ! reject. |
| (5) Metric | The 'distance' to the target (usually counted in hops). It is not used by recent kernels, but may be needed by routing daemons. |
| (6) Ref | Number of references to this route. (Not used in the Linux kernel.) |
| (7) Use | Count of lookups for the route. |
| (8) Iface | Interface to which packets for this route will be sent. |
| (9) irtt | Initial RTT (Round Trip Time). The kernel uses this to guess about the best TCP protocol parameters without waiting on (possibly slow) answers. |
| (10) Net/Mask | The destination network or destination host. |
| (11) dev | Interface to which packets for this route will be sent. |
| (12) proto | man ip /RTPROTO: redirect, kernel, boot, static, ra |
| (13) scope | man ip /SCOPE_WALUE: global, site, link, host |
| (14) src | The source address to prefer when sending to the destinations covered by the route prefix. |
| (15) default | Gateway address for all addresses without explicitly assigned gateways |
| (15) dev | Interface to which packets for this route will be sent. |
The important field for now:
- (1) Destination — The destination network or destination host.
- (2) Gateway — The gateway address or '*' if none set. Default means that packets will be sent via this gateway if there is no explicitly specified gateway for packet destination address.
- (3) Genmask — The netmask for the destination net; '255.255.255.255' for a host destination and '0.0.0.0' for the default route.
- (8) Iface — Interface to which packets for this route will be sent.
Which field from netstat and route correspond to which from ip route show is left as an exercise for you once more. Well, that was awfully big! Take a deep breath and let us move to the practice.
Now you will learn how to create pseudo-interface, assign address to it and change it state.
Do this
1: sudo aptitude install uml-utilities 2: sudo tunctl -t tap0 -u user1 3: ls -al /sys/devices/virtual/net/tap0 4: sudo ifconfig tap0 10.1.1.1 netmask 255.255.255.0 5: sudo ifconfig 6: sudo route 7: ping 10.1.1.1 -c 2 8: sudo ifconfig tap0 down 9: ping 10.1.1.1 -c 2 10: sudo ifconfig tap0 up 11: sudo ip a a 10.2.2.2/24 brd + dev tap0 12: sudo ifconfig 13: sudo route 14: ip a s 15: ip r s 16: ping 10.2.2.2 -c 2 17: sudo ip link set dev tap0 down 18: ip a s 19: ip r s 20: ping 10.2.2.2 -c 2 21: sudo tunctl -d tap0 22: ip a s 23: ls -al /sys/devices/virtual/net/tap0
What you should see
user1@vm1:~$ sudo aptitude install uml-utilities The following NEW packages will be installed: libfuse2{a} uml-utilities 0 packages upgraded, 2 newly installed, 0 to remove and 0 not upgraded. Need to get 0 B/205 kB of archives. After unpacking 737 kB will be used. Do you want to continue? [Y/n/?] Selecting previously deselected package libfuse2. (Reading database ... 39616 files and directories currently installed.) Unpacking libfuse2 (from .../libfuse2_2.8.4-1.1_amd64.deb) ... Selecting previously deselected package uml-utilities. Unpacking uml-utilities (from .../uml-utilities_20070815-1.1_amd64.deb) ... Processing triggers for man-db ... Setting up libfuse2 (2.8.4-1.1) ... Setting up uml-utilities (20070815-1.1) ... Starting User-mode networking switch: uml_switch. user1@vm1:~$ sudo tunctl -t tap0 -u user1 Set 'tap0' persistent and owned by uid 1000 user1@vm1:~$ ls -al /sys/devices/virtual/net/tap0 total 0 drwxr-xr-x 4 root root 0 Jul 11 05:33 . drwxr-xr-x 4 root root 0 Jul 11 05:33 .. -r--r--r-- 1 root root 4096 Jul 11 05:33 address -r--r--r-- 1 root root 4096 Jul 11 05:33 addr_len -r--r--r-- 1 root root 4096 Jul 11 05:33 broadcast -r--r--r-- 1 root root 4096 Jul 11 05:33 carrier -r--r--r-- 1 root root 4096 Jul 11 05:33 dev_id -r--r--r-- 1 root root 4096 Jul 11 05:33 dormant -r--r--r-- 1 root root 4096 Jul 11 05:33 duplex -r--r--r-- 1 root root 4096 Jul 11 05:33 features -rw-r--r-- 1 root root 4096 Jul 11 05:33 flags -r--r--r-- 1 root root 4096 Jul 11 05:33 group -rw-r--r-- 1 root root 4096 Jul 11 05:33 ifalias -r--r--r-- 1 root root 4096 Jul 11 05:33 ifindex -r--r--r-- 1 root root 4096 Jul 11 05:33 iflink -r--r--r-- 1 root root 4096 Jul 11 05:33 link_mode -rw-r--r-- 1 root root 4096 Jul 11 05:33 mtu -r--r--r-- 1 root root 4096 Jul 11 05:33 operstate -r--r--r-- 1 root root 4096 Jul 11 05:33 owner drwxr-xr-x 2 root root 0 Jul 11 05:33 power -r--r--r-- 1 root root 4096 Jul 11 05:33 speed drwxr-xr-x 2 root root 0 Jul 11 05:33 statistics lrwxrwxrwx 1 root root 0 Jul 11 05:33 subsystem -> ../../../../class/net -r--r--r-- 1 root root 4096 Jul 11 05:33 tun_flags -rw-r--r-- 1 root root 4096 Jul 11 05:33 tx_queue_len -r--r--r-- 1 root root 4096 Jul 11 05:33 type -rw-r--r-- 1 root root 4096 Jul 11 05:33 uevent user1@vm1:~$ sudo ifconfig tap0 10.1.1.1 netmask 255.255.255.0 user1@vm1:~$ sudo ifconfig eth0 Link encap:Ethernet HWaddr 08:00:27:d4:45:68 inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fed4:4568/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:64040 errors:0 dropped:0 overruns:0 frame:0 TX packets:44578 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:19663646 (18.7 MiB) TX bytes:25043918 (23.8 MiB) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:76 errors:0 dropped:0 overruns:0 frame:0 TX packets:76 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:6272 (6.1 KiB) TX bytes:6272 (6.1 KiB) tap0 Link encap:Ethernet HWaddr ee:d8:2e:f6:bc:f1 inet addr:10.1.1.1 Bcast:10.1.1.255 Mask:255.255.255.0 inet6 addr: fe80::ecd8:2eff:fef6:bcf1/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:1 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) user1@vm1:~$ sudo route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 10.0.2.0 * 255.255.255.0 U 0 0 0 eth0 10.1.1.0 * 255.255.255.0 U 0 0 0 tap0 default 10.0.2.2 0.0.0.0 UG 0 0 0 eth0 user1@vm1:~$ ping 10.1.1.1 -c 2 PING 10.1.1.1 (10.1.1.1) 56(84) bytes of data. 64 bytes from 10.1.1.1: icmp_req=1 ttl=64 time=0.070 ms 64 bytes from 10.1.1.1: icmp_req=2 ttl=64 time=0.027 ms --- 10.1.1.1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 1001ms rtt min/avg/max/mdev = 0.027/0.048/0.070/0.022 ms user1@vm1:~$ sudo ifconfig tap0 down user1@vm1:~$ ping 10.1.1.1 -c 2 PING 10.1.1.1 (10.1.1.1) 56(84) bytes of data. 64 bytes from 10.1.1.1: icmp_req=1 ttl=64 time=0.030 ms 64 bytes from 10.1.1.1: icmp_req=2 ttl=64 time=0.024 ms --- 10.1.1.1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.024/0.027/0.030/0.003 ms user1@vm1:~$ sudo ifconfig tap0 up user1@vm1:~$ sudo ip a a 10.2.2.2/24 brd + dev tap0 user1@vm1:~$ sudo ifconfig eth0 Link encap:Ethernet HWaddr 08:00:27:d4:45:68 inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fed4:4568/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:64088 errors:0 dropped:0 overruns:0 frame:0 TX packets:44609 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:19667480 (18.7 MiB) TX bytes:25049771 (23.8 MiB) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:84 errors:0 dropped:0 overruns:0 frame:0 TX packets:84 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:6944 (6.7 KiB) TX bytes:6944 (6.7 KiB) tap0 Link encap:Ethernet HWaddr ee:d8:2e:f6:bc:f1 inet addr:10.1.1.1 Bcast:10.1.1.255 Mask:255.255.255.0 inet6 addr: fe80::ecd8:2eff:fef6:bcf1/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:9 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) user1@vm1:~$ sudo route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 10.2.2.0 * 255.255.255.0 U 0 0 0 tap0 10.0.2.0 * 255.255.255.0 U 0 0 0 eth0 10.1.1.0 * 255.255.255.0 U 0 0 0 tap0 default 10.0.2.2 0.0.0.0 UG 0 0 0 eth0 user1@vm1:~$ ip a s 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:d4:45:68 brd ff:ff:ff:ff:ff:ff inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0 inet6 fe80::a00:27ff:fed4:4568/64 scope link valid_lft forever preferred_lft forever 12: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500 link/ether ee:d8:2e:f6:bc:f1 brd ff:ff:ff:ff:ff:ff inet 10.1.1.1/24 brd 10.1.1.255 scope global tap0 inet 10.2.2.2/24 brd 10.2.2.255 scope global tap0 inet6 fe80::ecd8:2eff:fef6:bcf1/64 scope link valid_lft forever preferred_lft forever user1@vm1:~$ ip r s 10.2.2.0/24 dev tap0 proto kernel scope link src 10.2.2.2 10.0.2.0/24 dev eth0 proto kernel scope link src 10.0.2.15 10.1.1.0/24 dev tap0 proto kernel scope link src 10.1.1.1 default via 10.0.2.2 dev eth0 user1@vm1:~$ ping 10.2.2.2 -c 2 PING 10.2.2.2 (10.2.2.2) 56(84) bytes of data. 64 bytes from 10.2.2.2: icmp_req=1 ttl=64 time=0.081 ms 64 bytes from 10.2.2.2: icmp_req=2 ttl=64 time=0.025 ms --- 10.2.2.2 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.025/0.053/0.081/0.028 ms user1@vm1:~$ sudo ip link set dev tap0 down user1@vm1:~$ ip a s 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:d4:45:68 brd ff:ff:ff:ff:ff:ff inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0 inet6 fe80::a00:27ff:fed4:4568/64 scope link valid_lft forever preferred_lft forever 12: tap0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN qlen 500 link/ether ee:d8:2e:f6:bc:f1 brd ff:ff:ff:ff:ff:ff inet 10.1.1.1/24 brd 10.1.1.255 scope global tap0 inet 10.2.2.2/24 brd 10.2.2.255 scope global tap0 user1@vm1:~$ ip r s 10.0.2.0/24 dev eth0 proto kernel scope link src 10.0.2.15 default via 10.0.2.2 dev eth0 user1@vm1:~$ ping 10.2.2.2 -c 2 PING 10.2.2.2 (10.2.2.2) 56(84) bytes of data. 64 bytes from 10.2.2.2: icmp_req=1 ttl=64 time=0.037 ms 64 bytes from 10.2.2.2: icmp_req=2 ttl=64 time=0.024 ms --- 10.2.2.2 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.024/0.030/0.037/0.008 ms user1@vm1:~$ sudo tunctl -d tap0 Set 'tap0' nonpersistent user1@vm1:~$ ip a s 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:d4:45:68 brd ff:ff:ff:ff:ff:ff inet 10.0.2.15/24 brd 10.0.2.255 scope global eth0 inet6 fe80::a00:27ff:fed4:4568/64 scope link valid_lft forever preferred_lft forever user1@vm1:~$ ls -al /sys/devices/virtual/net/tap0 ls: cannot access /sys/devices/virtual/net/tap0: No such file or directory user1@vm1:~$
Explanation
- Install package for working with pseudo-(virtual)-interfaces.
- Create pseudo-interface tap0.
- Prints out contents of virtual directory created for this interfaces, containing its settings and statistics.
- Add to tap0 an IP address 10.1.1.1/24.
- Print out current interface status.
- Print out current routing table entries. Notice that Linux automatically added new route for tap0.
- Tests tap0 by sending ICMP echo request packet to it.
- Brings tap0 to DOWN state.
- Tests tap0 by sending ICMP echo request packet to it again. There will be an Extra credit for you to explain why this still works despite intercase being down?
- Brings tap0 to UP state.
- Adds additional IP address 10.2.2.2/24 to tap0. ip a a is abbreviated version of ip addr add. What this little + means you will find out on your own in Extra credit.
- Prints out current interface status. Notice how ifconfig is unable to list new IP address added using ip utility. Why? More Extra credit.
- Print out current routing table. Notice that Linux automatically added one more route for tap0.
- Prints out current interface status using ip utility. You are able to see newly added address here.
- Prints our routing tables using ip utility.
- Tests net tap0 IP address by sending ICMP echo request packet to it.
- Brings tap0 to DOWN state.
- Prints out current interface status.
- Print out current routing table entries. Notice that tap0 routes are remove automatically.
- Tests net tap0 IP address by sending ICMP echo request packet to it. This works. Why?
- Removes pseudo-interface tap0.
- Prints out current interface status. tap0 is absent.
- Shows us that tap0 virtual directory is now gone as well.
Extra Credit
- Familiarize yourself with man ifconfig, man ip, man netstat, man ss.
- Why ping worked when tap0 was is in down state?
- What brd + means?
- Why ifconfig is unable to list new address you added using ip?
