Nobody likes to get paged. This is especially true of pages for mysterious network issues that have cleared up by the time you have a chance to look at them. This has happened to us very occasionally over the last few months.

We ran through the obvious culprits - were there error counters we missed? Usage spikes? There was nothing visible. So we tried to figure out what common element there was, if any, for the hosts we were getting paged for.

One clue was that all the hosts were on the same physical layer 2 network, but on different layer 3 networks. We also weren’t seeing any failures with IPv6, only IPv4.

From one manual run of ping we saw output like:

From icmp_seq=1 Redirect Host(New nexthop:
27 packets transmitted, 27 received, +1 errors, 0% packet loss, time 26018ms

So where does this “Redirect Host” message come from?

When host A sends traffic to host B, if host A and B aren’t on the same network, i.e. they can’t talk directly to each other, host A will send the traffic to host B via a third party, a router. If you’ve ever manually configured a network and had to enter a gateway IP address, it is the router that this gateway address belongs to.

If a router has the gateway IPs for host A and host B on the same physical interface, it may send what is called an ICMP redirect packet when host A sends traffic for host B via the router. The redirect packet lets host A know that it can contact host B directly without going through the router. This specific type of redirect is a “shared media” redirect and is described in RFC1620. Only the gateway router is allowed to send this message. In Linux, sending and receiving this message is controlled by the ip sysctl settings send_redirects, accept_redirects, and shared_media.

Seeing that error was odd, but was not a smoking gun. Eventually we captured output similar to:

CMD: /usr/bin/ping -n -U -W 10 -c 5
Output: PING ( 56(84) bytes of data.
Output: From icmp_seq=1 Redirect Host(New nexthop:
Output: From icmp_seq=1 Redirect Host(New nexthop:
Output: 64 bytes from icmp_seq=1 ttl=63 time=0.490 ms
Output: 64 bytes from icmp_seq=2 ttl=63 time=0.613 ms
Output: 64 bytes from icmp_seq=3 ttl=63 time=0.565 ms
Output: From icmp_seq=4 Redirect Host(New nexthop:
Output: --- ping statistics ---
Output: 4 packets transmitted, 3 received, +2 errors, 25% packet loss,
Output: rtt min/avg/max/mdev = 0.490/0.556/0.613/0.050 ms
PING CRITICAL - Packet loss = 25%, RTA = 0.56
ms|rta=0.556000ms;100.000000;500.000000;0.000000 pl=25%;2;6;0
100.000000:2% 500.000000:6%

This reported packet loss of 25% must have been why we were seeing problems.

After we turned off sending redirects on our routers, which was on by default, we looked into why this was occurring within ping.

One odd thing about ping is that if a count of packets is specified with -c, the count specifies the number of packets to receive, not the number of packets to send. In the above output, receiving a redirect as the 5th packet meant there was no response received for the last probe sent.

By code inspection and pulling up ping in a debugger, we found that a message for the ICMP redirect was being inserted into the socket error queue. The socket error queue is read by calling the function recvmsg with the MSG_ERRQUEUE flag. We tried patching ping to not increment the number of errors when a message of type ICMP_REDIRECT was received, and that worked.

However, it was curious that this patch was needed. By testing other distributions we found that there was no error received for ICMP redirects, and that if we replaced the stock CentOS 7 kernel with an alternative one, the errors went away. A ticket has been filed with CentOS.