iptables prevents me from connecting

I'm using Ubuntu 18.04 and my current iptables rules are from this question; iptables -L gives:

Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
REJECT all -- anywhere anywhere reject-with icmp-port-unreachable
Chain FORWARD (policy DROP)
target prot opt source destination
REJECT all -- anywhere anywhere reject-with icmp-port-unreachable
Chain OUTPUT (policy ACCEPT)
target prot opt source destination 

and with these I can't connect anywhere. I have to flush the list and accept again (from scratch) to restore connectivity. Why are these not working?

1 Answer

You need to allow the local loopback interface:

sudo iptables -A INPUT -i lo -j ACCEPT

Sometimes inter-process communications takes place over the loopback interface. See here for more information about the loopback interface.

EDIT: So I have exactly this on one of my test computers:

doug@s17:~$ sudo iptables -v -x -n -L
Chain INPUT (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination 94 9843 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0 5093 6056565 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED 8 613 REJECT all -- * * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
Chain FORWARD (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination 0 0 REJECT all -- * * 0.0.0.0/0 0.0.0.0/0 reject-with icmp-port-unreachable
Chain OUTPUT (policy ACCEPT 3904 packets, 321224 bytes) pkts bytes target prot opt in out source destination

Now, if I wanted to gain further insight into the rejected packets, I could also log them:

sudo iptables --insert INPUT 3 --jump LOG --log-prefix "INPUT-REJECT:" --log-level info

And then (after some more rejects) look at /var/log/syslog:

Dec 12 14:18:42 s17 kernel: [2007598.577383] INPUT-REJECT:IN=ens5 OUT= MAC=ff:ff:ff:ff:ff:ff:00:21:9b:f9:21:26:08:00 SRC=192.168.111.101 DST=255.255.255.255 LEN=42 TOS=0x00 PREC=0x00 TTL=1 ID=4970 PROTO=UDP SPT=55705 DPT=3289 LEN=22
Dec 12 14:18:42 s17 kernel: [2007598.695347] INPUT-REJECT:IN=ens5 OUT= MAC=ff:ff:ff:ff:ff:ff:00:21:9b:f9:21:26:08:00 SRC=192.168.111.101 DST=255.255.255.255 LEN=42 TOS=0x00 PREC=0x00 TTL=1 ID=4971 PROTO=UDP SPT=55708 DPT=3289 LEN=22
Dec 12 14:18:43 s17 kernel: [2007599.014201] INPUT-REJECT:IN=ens5 OUT= MAC=ff:ff:ff:ff:ff:ff:00:21:9b:f9:21:26:08:00 SRC=192.168.111.101 DST=255.255.255.255 LEN=42 TOS=0x00 PREC=0x00 TTL=1 ID=4972 PROTO=UDP SPT=55712 DPT=3289 LEN=22

And I observe that they are just broadcast packets from this computer that I am typing on now.