ipv6: using source address from wrong interface

ipv6: using source address from wrong interface

[Brian J. Murrell]

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Hi,

I have a situation here on a Linux 3.10.36 OpenWRT router where I have
two IPv6 interfaces:

6in4-henet Link encap:IPv6-in-IPv4  
          inet6 addr: 2001:613:1c:28f::2/64 Scope:Global
          inet6 addr: fe80::587b:2005/128 Scope:Link
          UP POINTOPOINT RUNNING NOARP  MTU:1280  Metric:1
          RX packets:98181547 errors:0 dropped:0 overruns:0 frame:0
          TX packets:52168025 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:113677566440 (105.8 GiB)  TX bytes:5441656803 (5.0 GiB)

6to4-foo6 Link encap:IPv6-in-IPv4  
          inet6 addr: ::88.123.32.5/128 Scope:Compat
          inet6 addr: 2002:587b:2005::1/16 Scope:Global
          UP RUNNING NOARP  MTU:1280  Metric:1
          RX packets:54095 errors:6 dropped:0 overruns:0 frame:0
          TX packets:107525 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:7650028 (7.2 MiB)  TX bytes:11379920 (10.8 MiB)

The LAN interface on the other side of the router is:

br-lan    Link encap:Ethernet  HWaddr C0:A0:BB:ED:38:D1  
          inet addr:10.75.22.253  Bcast:10.75.22.255  Mask:255.255.255.0
          inet6 addr: 2001:613:1d:28f::1/64 Scope:Global
          inet6 addr: fe80::c2a0:bbff:feed:38d1/64 Scope:Link
          inet6 addr: 2002:587b:2005::1/60 Scope:Global
          inet6 addr: fd31:aeb1:48df::1/60 Scope:Global
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:867428811 errors:0 dropped:0 overruns:0 frame:0
          TX packets:693644720 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:429976183525 (400.4 GiB)  TX bytes:749419207609 (697.9 GiB)

But when the router needs to generate an ICMP6 "packet too big" message
back to the sender, the wrong interface's address is being used as the
source address.  Witness tcpdump on the 6in4-henet interface:

11:52:00.206228 IP6 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548 > 2001:8d8:1001:27f:2736:6506:2744:808.443: Flags [S], seq 3466582922, win 28800, options [mss 1440,sackOK,TS val 4004937347 ecr 0,nop,wscale 7], length 0
11:52:00.310829 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [S.], seq 250830792, ack 3466582923, win 14400, options [mss 1440,nop,wscale 7], length 0
11:52:00.314706 IP6 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548 > 2001:8d8:1001:27f:2736:6506:2744:808.443: Flags [.], ack 1, win 225, length 0
11:52:00.449646 IP6 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548 > 2001:8d8:1001:27f:2736:6506:2744:808.443: Flags [P.], seq 1:107, ack 1, win 225, length 106
11:52:00.551007 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], ack 107, win 113, length 0
11:52:00.662576 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 1:1421, ack 107, win 113, length 1420
11:52:00.662867 IP6 2002:587b:2005::1 > 2001:8d8:1001:27f:2736:6506:2744:808: ICMP6, packet too big, mtu 1280, length 1240
11:52:00.663178 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 1421:2841, ack 107, win 113, length 1420
11:52:00.663380 IP6 2002:587b:2005::1 > 2001:8d8:1001:27f:2736:6506:2744:808: ICMP6, packet too big, mtu 1280, length 1240
11:52:00.663508 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 2841:4261, ack 107, win 113, length 1420
11:52:00.663689 IP6 2002:587b:2005::1 > 2001:8d8:1001:27f:2736:6506:2744:808: ICMP6, packet too big, mtu 1280, length 1240
11:52:00.663793 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 4261:4321, ack 107, win 113, length 60
11:52:00.667654 IP6 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548 > 2001:8d8:1001:27f:2736:6506:2744:808.443: Flags [.], ack 1, win 234, length 0
11:52:01.382115 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 1:1421, ack 107, win 113, length 1420
11:52:01.382349 IP6 2002:587b:2005::1 > 2001:8d8:1001:27f:2736:6506:2744:808: ICMP6, packet too big, mtu 1280, length 1240
11:52:02.826238 IP6 2001:8d8:1001:27f:2736:6506:2744:808.443 > 2001:613:1d:28f:224:d7ff:fe7b:1f24.55548: Flags [.], seq 1:1421, ack 107, win 113, length 1420
11:52:02.826471 IP6 2002:587b:2005::1 > 2001:8d8:1001:27f:2736:6506:2744:808: ICMP6, packet too big, mtu 1280, length 1240

Notice that interface 6in4-henet is being used to make the connection to
2001:8d8:1001:27f:2736:6506:2744:808 from
2001:613:1d:28f:224:d7ff:fe7b:1f24 however when the router needs to send
an ICMP6 packet, it is using the source address from the 6to4-foo6
interface even though the packet was received on and needs to be sent
out on the 6in4-henet interface.

Why would this be?

Cheers,
b.


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