<div dir="ltr"><div><div style="font-size:small" class="gmail_default">So here is the situation.</div><div style="font-size:small" class="gmail_default">A Linux machine refuses to communicate with the outside world.</div><div style="font-size:small" class="gmail_default"><br></div><div style="font-size:small" class="gmail_default">When using an IP address, things work well.</div><div style="font-size:small" class="gmail_default"><br></div><div style="font-size:small" class="gmail_default"><span style="font-family:monospace"><span style="color:rgb(0,0,0);background-color:rgb(255,255,255)">ping -c 4 8.8.8.8
</span><br>PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
<br>64 bytes from <a href="http://8.8.8.8">8.8.8.8</a>: icmp_seq=1 ttl=115 time=36.7 ms
<br>64 bytes from <a href="http://8.8.8.8">8.8.8.8</a>: icmp_seq=2 ttl=115 time=44.2 ms
<br>64 bytes from <a href="http://8.8.8.8">8.8.8.8</a>: icmp_seq=3 ttl=115 time=42.5 ms
<br>64 bytes from <a href="http://8.8.8.8">8.8.8.8</a>: icmp_seq=4 ttl=115 time=35.3 ms
<br>
<br>--- 8.8.8.8 ping statistics ---
<br>4 packets transmitted, 4 received, 0% packet loss, time 3005ms
<br>rtt min/avg/max/mdev = 35.331/39.683/44.225/3.765 ms<br></span></div><div style="font-size:small" class="gmail_default"><span style="font-family:monospace"><br></span></div><div style="font-size:small" class="gmail_default">But, when doing a DNS query, 127.0.0.53 times out:</div><div style="font-size:small" class="gmail_default"><br></div><div style="font-size:small" class="gmail_default"><span style="font-family:monospace"><span style="color:rgb(0,0,0);background-color:rgb(255,255,255)">$ dig <a href="http://google.com">google.com</a>
</span><br>;; communications error to 127.0.0.53#53: timed out
<br>;; communications error to 127.0.0.53#53: timed out
<br>;; communications error to 127.0.0.53#53: timed out <br>...</span></div><div style="font-size:small" class="gmail_default"><span style="font-family:monospace">;; no servers could be reached<br></span></div><br clear="all"></div><div><span class="gmail_default" style="font-size:small">When I explicitly specify a DNS server IP address, it resolves:</span></div><div><br></div><div><div style="font-size:small" class="gmail_default"><span style="font-family:monospace"><span style="color:rgb(0,0,0);background-color:rgb(255,255,255)">dig @<a href="http://1.1.1.1">1.1.1.1</a> <a href="http://google.com">google.com</a>
</span><br>
<br>;; Got answer:
<br>;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56116
<br>;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1
<br>
<br>;; ANSWER SECTION:
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.113
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.102
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.101
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.138
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.139
<br><a href="http://google.com">google.com</a>. 153 IN A 142.250.139.100
<br>
<br>;; Query time: 49 msec
<br>;; SERVER: 1.1.1.1#53(1.1.1.1) (UDP)
<br>;; WHEN: Tue Feb 10 20:08:30 EST 2026
<br>;; MSG SIZE rcvd: 135<br></span></div><br></div><div><br></div><div>A<span class="gmail_default" style="font-size:small">nd when checking who is listening on port 53</span></div><div><span class="gmail_default" style="font-size:small"><br></span></div><span style="font-family:monospace"><span style="color:rgb(0,0,0);background-color:rgb(255,255,255)">sudo netstat -ltunp | grep -w 53 | grep -v 54
</span><br>tcp<span class="gmail_default" style="font-size:small"></span><span class="gmail_default" style="font-size:small"> </span>0<span class="gmail_default" style="font-size:small"> </span>0 <a href="http://127.0.0.53:53">127.0.0.53:53</a><span class="gmail_default" style="font-size:small"> </span>0.0.0.0:*<span class="gmail_default" style="font-size:small"> </span>LISTEN<span class="gmail_default" style="font-size:small"> </span>2056751/systemd-res
<br>udp<span class="gmail_default" style="font-size:small"></span><span class="gmail_default" style="font-size:small"> </span>0<span class="gmail_default" style="font-size:small"> </span>0 <a href="http://127.0.0.53:53">127.0.0.53:53</a><span class="gmail_default" style="font-size:small"> </span>0.0.0.0:*<span class="gmail_default" style="font-size:small"></span><span class="gmail_default" style="font-size:small"> </span>2056751/systemd-res<br></span><div><br></div><div>N<span class="gmail_default" style="font-size:small">ote that systemd-resolved shows "LISTEN" on the tcp port, but not the udp one.</span></div><div><span class="gmail_default" style="font-size:small"><br></span></div><div><span class="gmail_default" style="font-size:small">Of course, I can restart systemd-resolved, or reboot the machine.</span></div><div><span class="gmail_default" style="font-size:small">However, I really want to diagnose what the issue is first.</span></div><div><span class="gmail_default" style="font-size:small">But I want to diagnose the issue first, for future cases.</span></div><div><span class="gmail_default" style="font-size:small"><br></span></div><div><span class="gmail_default" style="font-size:small">So my questions are:</span></div><div><span class="gmail_default" style="font-size:small">Why would the above happen?</span></div><div><span class="gmail_default" style="font-size:small">And how to prevent it from happening again? </span></div><div><span class="gmail_signature_prefix">-- </span></div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Khalid M. Baheyeldin</div></div>