Weird problem with pinging outside my network

hanj · Veteran Joined: 19 Aug 2003 Posts: 1500

Not sure when it started happening, but today I noticed that all of my icmp ping requests going outside of my network suddenly is a black hole. 100% loss, with no response. Internet and traffic to sites work fine.

While internal, I can ping any device internally. I can ping the inside and outside of my gentoo firewall interfaces. But as soon as I go one more step out, nothing. Loaded new kernel, double checked iptables to ensure icmp is allowed, it is (and I haven't touched the script in a while). This includes any device on my network, so windows, etc.. can't ping outside.. so I'm guessing the broken point is the firewall. Again, internet and all normal traffic is fine. I just need to ping something today and noticed this issue.

I'm also pinging devices that I know allow pings - including 4.2.2.2 and 8.8.8.8. I've been pinging by IP (not hostname)

For example...

krinn · Watchman Joined: 02 May 2003 Posts: 7470

well, the easy test is no firewall and see if ping works.
if it works, you have the answer
if it doesn't work, we will need more infos

mike155 · Posted: Tue May 28, 2019 10:57 pm Post subject:

Log in to your firewall machine and start tcpdump on the outgoing interface:

hanj · Veteran Joined: 19 Aug 2003 Posts: 1500

Hu · Administrator Joined: 06 Mar 2007 Posts: 23000

You might have iptables configured with an SNAT target that forces the outgoing IP address to be a value set by SNAT, rather than the value currently correct for your network interface. If your public IP once matched the SNAT value, and now does not, that could explain the symptoms you report. Inspect your nat table in iptables-save to see if this is the cause.