I'm trying to diagnose a connectivity problem on a headless ubuntu (16.04) server so I have a second ethernet interface (call it "eth1"), and I can use that to start a terminal ssh session from my laptop. What's the best way to disable outbound connections on that interface so that the terminal still works, but outbound requests will only go to the other interface that I am debugging?
I feel like a ufw rule to drop outbound packets is not the right answer, because the tcp/ip stack will still try to use the known good eth1 interface and it will just have it's packets discarded, and not necessarily reroute to eth0 where I want the traffic to go.
output of ip a s
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp6s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
link/ether 40:8d:5c:14:04:e3 brd ff:ff:ff:ff:ff:ff
3: wlp7s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000
link/ether 48:51:b7:84:10:66 brd ff:ff:ff:ff:ff:ff
4: enx0050b6294caf: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 00:50:b6:29:4c:af brd ff:ff:ff:ff:ff:ff
inet 10.2.10.102/24 brd 10.2.10.255 scope global dynamic enx0050b6294caf
valid_lft 4183sec preferred_lft 4183sec
inet6 fe80::b104:ac65:6a10:caf1/64 scope link
valid_lft forever preferred_lft forever
5: enx0050b6b50965: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 00:50:b6:b5:09:65 brd ff:ff:ff:ff:ff:ff
inet 10.3.10.100/24 brd 10.3.10.255 scope global dynamic enx0050b6b50965
valid_lft 6832sec preferred_lft 6832sec
inet6 fe80::5809:6749:217a:20dc/64 scope link
valid_lft forever preferred_lft forever
6: enx0050b6b50963: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 00:50:b6:b5:09:63 brd ff:ff:ff:ff:ff:ff
inet 10.1.10.207/24 brd 10.1.10.255 scope global enx0050b6b50963
valid_lft forever preferred_lft forever
inet6 2603:3024:20b:2200:2565:531b:cc3c:2128/64 scope global temporary dynamic
valid_lft 318817sec preferred_lft 18007sec
inet6 2603:3024:20b:2200:250:b6ff:feb5:963/64 scope global mngtmpaddr dynamic
valid_lft 318817sec preferred_lft 318817sec
inet6 fe80::250:b6ff:feb5:963/64 scope link
valid_lft forever preferred_lft forever
7: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
link/ether 02:42:9a:df:18:e8 brd ff:ff:ff:ff:ff:ff
inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
valid_lft forever preferred_lft forever
output of ip r s
default via 10.2.10.10 dev enx0050b6294caf proto static metric 100
default via 10.3.10.10 dev enx0050b6b50965 proto static metric 101
default via 10.1.10.1 dev enx0050b6b50963 metric 800
10.1.10.0/24 dev enx0050b6b50963 proto kernel scope link src 10.1.10.207
10.2.10.0/24 dev enx0050b6294caf proto kernel scope link src 10.2.10.102 metric 100
10.3.10.0/24 dev enx0050b6b50965 proto kernel scope link src 10.3.10.100 metric 100
169.254.0.0/16 dev enx0050b6294caf scope link metric 1000
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown
