We have an interesting case with one of our carriers. Looks like we have worked around it for now, but it is rather odd as they are requiring non standard BGP TCP/IP in links to them.
They are requiring us to send all the BGP TCP packets with a TTL of 1
What is interesting is that some of the big routers do indeed do this, but doing so is against the recommendations for TCP/IP which recommends a TTL of 64. BGP itself makes no mention of TTL. There are Internet standards that say the TTL must be at least the Internet diameter even. So naturally, our BGP does in fact follow this standard and uses a TTL of 64.
Of course, using TTL of 1 was a silly thing anyway as anyone could spoof BGP with a TTL of 1 by setting it to a suitable higher value, though if the reply was TTL of 1 they do not get far. The issue that came up is anyone can spoof a convincing TCP RST with a TTL of one and shut down BGP sessions. This problem is now recognised in other Internet standards which document TTL security where one sets a TTL of 255 and the far end checks it is still 255. Remotely spoofing a TTL of 255 is impossible without compromising the local routers somehow. So that works. Indeed our routers support TTL security.
It seems this is some fire-walling rule, and to be honest I have never seen anyone fire-walling based on TTL. It is not clear what they are trying to protect against. They allow L2TP with normal TTLs.
A simpler firewall would be to do the same as other carriers and have access lists covering which of our IPs can talk to which of their IPs, and not fire-walling on TTL.
This is the same bunch that allowed MAC spoofing on PPPoE links to disrupt and even monitor other people's DSL lines. Thankfully they are fixing that.
It seems however they are insisting that any future services we buy must use this non standard BGP to connect to them.
It is very brave of them.
I guess following the standards is a key factor in deciding which suppliers we will use for new services.
P.S. FireBrick have, of course, modified the TCP stack and BGP and config on the FireBrick BGP routers to support this non standard mode of operation as well as standard TTL security.