Public bug reported: We have discovered what appears to be a bug with the handling of ICMP6 router solicitations on tun interfaces in the Linux kernel. In particular, if an application is not immediately bound to the tun interface, router solicitations are never sent out of the interface. This issue can be replicated easily using the attached scripts.
First, a tun interface is created with permissions for the local user, sudo is used so the script will prompt for the user password. $ bash rftun0_create.sh rftun0 created 16: rftun0: <POINTOPOINT,MULTICAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 500 link/none Then the interface is configured, given an address, and brought "up" however since there is no application bound to the interface yet there is no carrier. $ bash rftun0_up.sh net.ipv6.conf.rftun0.forwarding = 0 net.ipv6.conf.rftun0.accept_ra = 1 net.ipv6.conf.rftun0.autoconf = 1 net.ipv6.conf.rftun0.accept_ra_rt_info_max_plen = 128 net.ipv6.conf.rftun0.accept_ra_rtr_pref = 1 net.ipv6.conf.rftun0.router_solicitation_delay = 1 net.ipv6.conf.rftun0.router_solicitation_interval = 2 net.ipv6.conf.rftun0.router_solicitations = 200 rftun0 up 16: rftun0: <NO-CARRIER,POINTOPOINT,MULTICAST,NOARP,UP> mtu 1400 qdisc pfifo_fast state DOWN group default qlen 500 link/none inet6 fe80::600:4339:125f:760c/64 scope link valid_lft forever preferred_lft forever A simple client is then run to bind to the tun interface and read the first three packets. If things are going well we should see 3 router solicitations, however we instead see no traffic. $ python simple_tun_client.py Timed out... Timed out... Timed out... If we bring the interface down and try again with no delay between bringing the interface up and binding the application we see things work as expected. $ bash rftun0_down.sh rftun0 down 16: rftun0: <POINTOPOINT,MULTICAST,NOARP> mtu 1400 qdisc pfifo_fast state DOWN group default qlen 500 link/none $ bash rftun0_up.sh && python simple_tun_client.py net.ipv6.conf.rftun0.forwarding = 0 net.ipv6.conf.rftun0.accept_ra = 1 net.ipv6.conf.rftun0.autoconf = 1 net.ipv6.conf.rftun0.accept_ra_rt_info_max_plen = 128 net.ipv6.conf.rftun0.accept_ra_rtr_pref = 1 net.ipv6.conf.rftun0.router_solicitation_delay = 1 net.ipv6.conf.rftun0.router_solicitation_interval = 2 net.ipv6.conf.rftun0.router_solicitations = 200 rftun0 up 16: rftun0: <NO-CARRIER,POINTOPOINT,MULTICAST,NOARP,UP> mtu 1400 qdisc pfifo_fast state DOWN group default qlen 500 link/none inet6 fe80::600:1512:6243:38fe/64 scope link valid_lft forever preferred_lft forever Read packet from tunnel. 48 bytes. 6000000000083afffe8000000000000006001512624338feff0200000000000000000000000000028500c6e300000000 Read packet from tunnel. 48 bytes. 6000000000083afffe8000000000000006001512624338feff0200000000000000000000000000028500c6e300000000 Read packet from tunnel. 48 bytes. 6000000000083afffe8000000000000006001512624338feff0200000000000000000000000000028500c6e300000000 Furthermore, I can rerun the simple client and continue to receive router solicitations. Attempting the same procedure again with a small delay replicates the original issue. Notice the sleep statement in between the rftun0_up script and the simple tun client. $ bash rftun0_down.sh rftun0 down 16: rftun0: <POINTOPOINT,MULTICAST,NOARP> mtu 1400 qdisc pfifo_fast state DOWN group default qlen 500 link/none $ bash rftun0_up.sh && sleep 5 && python simple_tun_client.py net.ipv6.conf.rftun0.forwarding = 0 net.ipv6.conf.rftun0.accept_ra = 1 net.ipv6.conf.rftun0.autoconf = 1 net.ipv6.conf.rftun0.accept_ra_rt_info_max_plen = 128 net.ipv6.conf.rftun0.accept_ra_rtr_pref = 1 net.ipv6.conf.rftun0.router_solicitation_delay = 1 net.ipv6.conf.rftun0.router_solicitation_interval = 2 net.ipv6.conf.rftun0.router_solicitations = 200 rftun0 up 16: rftun0: <NO-CARRIER,POINTOPOINT,MULTICAST,NOARP,UP> mtu 1400 qdisc pfifo_fast state DOWN group default qlen 500 link/none inet6 fe80::600:25f2:67d0:37fc/64 scope link valid_lft forever preferred_lft forever Timed out... Timed out... Timed out... This test can be repeated as necessary. On my system, sleep times of 2 seconds or larger seem to always produce the issue, while sleep times of 1 second seem to work most of the time. ** Affects: linux (Ubuntu) Importance: Undecided Status: Incomplete ** Tags: kernel-bug ** Attachment added: "Contains the scripts used in the writeup" https://bugs.launchpad.net/bugs/1533304/+attachment/4548681/+files/scripts.tgz -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1533304 Title: Kernel Bug: IPv6 Neighbor Discovery To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1533304/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs