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Asked: 2019-08-12 08:44:56 +0800 CST

Does azure block INCOMING port 25 to VM?

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Added 443 and 25 to allow for inbound rule to VM Added 443 and 25 to allow for inbound rule to VM

iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A INPUT -p tcp -m tcp --dport 25 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 443 -j ACCEPT

Setup a listener on the vm

sudo nc -l 25

From external computer

telnet myVMip 25

Connection times out.

When doing the same procedure for 443 it works fine. This leads me to believe port 25 is blocked by azure itself?

linux

1 Answers

  1. Best way to troubleshoot this issue is usage of the traceroute with tcp packets. Use this command to check where the packets are blocked:

    traceroute -T -p 25 -n <myVMip>

    The next step - run the tcpdump on the VM and check incoming packets. The tcpdump captures packets before firewall.

    tcpdump -ni <iface> 'port 25'

    So, if you see the packets in the dump, but nothing works, then check the firewall. By default the iptables command lists only the filter table. Better use the iptables-save -c command to show the full rule set with rule counters.

    Then check, the port is really listened. Use the netstat -tlnp or ss -tlnp 'sport == :25' commands. You should see the listened socket and your program.

    If all steps above are done, but the service doesn't work, run the strace -p <pid> command to check, the service really gets data from sockets.

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