Get local network interface addresses using only proc?

Question

How can I obtain the (IPv4) addresses for all network interfaces using only proc? After some extensive investigation I've discovered the following:

1. ifconfig makes use of SIOCGIFADDR, which requires open sockets and advance knowledge of all the interface names. It also isn't documented in any manual pages on Linux.
2. proc contains /proc/net/dev, but this is a list of interface statistics.
3. proc contains /proc/net/if_inet6, which is exactly what I need but for IPv6.
4. Generally interfaces are easy to find in proc, but actual addresses are very rarely used except where explicitly part of some connection.
5. There's a system call called getifaddrs, which is very much a "magical" function you'd expect to see in Windows. It's also implemented on BSD. However it's not very text-oriented, which makes it difficult to use from non-C languages.

Answer 1

/proc/net/fib_trie holds the network topography

To simply print the addresses of all adapters:

$ awk '/32 host/ { print f } {f=$2}' <<< "$(</proc/net/fib_trie)" 127.0.0.1 192.168.0.5 192.168.1.14 

To determine the adapter of those addresses (a) consult the adapters' destination networks from /proc/net/route, (b) match those networks with the ones of /proc/net/fib_trie and (c) print the corresponding /32 host addresses listed under those networks.

Again no python unfortunately, but a quite awky bash approach:

#!/bin/bash  ft_local=$(awk '$1=="Local:" {flag=1} flag' <<< "$(</proc/net/fib_trie)")  for IF in $(ls /sys/class/net/); do     networks=$(awk '$1=="'$IF'" && $3=="00000000" && $8!="FFFFFFFF" {printf $2 $8 "\n"}' <<< "$(</proc/net/route)" )     for net_hex in $networks; do             net_dec=$(awk '{gsub(/../, "0x& "); printf "%d.%d.%d.%d\n", $4, $3, $2, $1}' <<< $net_hex)             mask_dec=$(awk '{gsub(/../, "0x& "); printf "%d.%d.%d.%d\n", $8, $7, $6, $5}' <<< $net_hex)             awk '/'$net_dec'/{flag=1} /32 host/{flag=0} flag {a=$2} END {print "'$IF':\t" a "\n\t'$mask_dec'\n"}' <<< "$ft_local"     done done  exit 0 

output:

eth0:     192.168.0.5           255.255.255.0  lo:       127.0.0.1           255.0.0.0  wlan0:    192.168.1.14           255.255.255.0 

Known limitation:

This approach does not work reliably for host addresses that share the network with other host addresses. This loss of network uniqueness makes it impossible to determine the correct host address from fib_trie as the order of those addresses does not necessarily match the order of networks of route.

Having said that, I'm not sure why you would want multiple host addresses belonging to the same network in first place. So in most use cases this approach should work just fine.

Answer 2

You may find the output of ip addr show easier to parse than output from other tools:

$ 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 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000     link/ether 00:24:1d:ce:47:05 brd ff:ff:ff:ff:ff:ff     inet 192.168.0.121/24 brd 192.168.0.255 scope global eth0     inet6 fe80::224:1dff:fece:4705/64 scope link        valid_lft forever preferred_lft forever 3: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN qlen 1000     link/ether 00:24:1d:ce:35:d5 brd ff:ff:ff:ff:ff:ff 4: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN     link/ether 92:e3:6c:08:1f:af brd ff:ff:ff:ff:ff:ff     inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0     inet6 fe80::90e3:6cff:fe08:1faf/64 scope link        valid_lft forever preferred_lft forever 

Another option is the file /proc/net/tcp. It shows all currently-open TCP sessions, which is different than what you asked for, but might be Good Enough.

$ cat tcp   sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode    0: 00000000:0050 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 13536 1 ffff88019f0a1380 300 0 0 2 -1    1: 00000000:1355 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 19877854 1 ffff880016e69380 300 0 0 2 -1    2: 017AA8C0:0035 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 13633 1 ffff88019f0a1a00 300 0 0 2 -1    3: 00000000:0016 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 8971 1 ffff88019f0a0000 300 0 0 2 -1    4: 0100007F:0277 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 12952880 1 ffff880030e30680 300 0 0 2 -1    5: 00000000:0539 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 14332 1 ffff88019f0a2080 300 0 0 2 -1    6: 00000000:C000 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 14334 1 ffff88019f0a2700 300 0 0 2 -1    7: 0100007F:0A44 00000000:0000 0A 00000000:00000000 00:00000000 00000000   119        0 51794804 1 ffff880016e6a700 300 0 0 2 -1    8: 7900A8C0:B094 53D50E48:01BB 01 00000000:00000000 00:00000000 00000000  1000        0 64877487 1 ffff880100502080 23 4 16 4 -1    9: 7900A8C0:9576 537F7D4A:01BB 06 00000000:00000000 03:00000E5D 00000000     0        0 0 3 ffff880100c84600   10: 7900A8C0:CC84 0CC181AE:01BB 01 00000000:00000000 00:00000000 00000000  1000        0 61775908 1 ffff880198715480 35 4 11 4 -1 $ irb irb(main):001:0> [0x79, 0x00, 0xa8, 0xc0] => [121, 0, 168, 192] 

My IP is 192.168.0.121; note the funny arithmetic to make it come out right. :)