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I have two VM's in azure with different public IP's and whose private IP's are :

10.10.1.9
10.10.1.6

When i do a telnet with following command from the server 10.10.1.6, i get the a error:

telnet 10.10.1.9 2181
Trying 10.10.1.9...
telnet: connect to address 10.10.1.9: No route to host

when i do a tcpdump in 10.10.1.9 side, i get the following log:

#tcpdump -i eth0 port 2181
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
07:55:35.530270 IP 10.10.1.6.55910 > 10.10.1.9.eforward: Flags [S], seq 1018543857, win 14600, options [mss 1418,sackOK,TS val 181360935 ecr 0,nop,wscale 7], length 0

Same time i also do a tcpdump on 10.10.1.6 side while i do a telnet from 10.10.1.6 to 10.10.1.9

tcpdump -i eth0 port 2181
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
07:55:57.970696 IP 10.10.1.6.55910 > 10.10.1.9.eforward: Flags [S], seq 1018543857, win 14600, options [mss 1460,sackOK,TS val 181360935 ecr 0,nop,wscale 7], length 0

**tcpdump on 10.10.1.9 with arp **

#tcpdump -i eth0 port 2181 or arp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
08:00:18.356153 IP 10.10.1.6.55944 > 10.10.1.9.eforward: Flags [S], seq 3337054296, win 14600, options [mss 1418,sackOK,TS val 181643770 ecr 0,nop,wscale 7], length 0
08:00:42.294801 ARP, Request who-has 10.10.1.6 tell 10.10.1.9, length 28
08:00:42.295859 ARP, Reply 10.10.1.6 is-at 12:34:56:78:9a:bc (oui Unknown), length 28

tcpdump on 10.10.1.6

tcpdump -i eth0 port 2181 or arp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
08:00:40.805565 IP 10.10.1.6.55944 > 10.10.1.9.eforward: Flags [S], seq 3337054296, win 14600, options [mss 1460,sackOK,TS val 181643770 ecr 0,nop,wscale 7], length 0
08:00:45.805204 ARP, Request who-has 10.10.1.9 tell 10.10.1.6, length 28
08:00:45.805721 ARP, Reply 10.10.1.9 is-at 12:34:56:78:9a:bc (oui Unknown), length 28
08:02:04.752283 ARP, Request who-has 10.10.1.9 tell 10.10.1.6, length 28
08:02:04.753141 ARP, Reply 10.10.1.9 is-at 12:34:56:78:9a:bc (oui Unknown), length 28

Sequence of run : First i ran tcpdumps on both 10.10.1.9 and 10.10.1.10 and then tried doing telnet from 10.10.1.10.

arp -a on 10.10.1.9

#arp -a
? (10.10.1.7) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.4) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.1) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.8) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.10) at <incomplete> on eth0
? (10.10.1.11) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.6) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.5) at 12:34:56:78:9a:bc [ether] on eth0

arp -a on 10.10.1.6

#arp -a
? (10.10.1.1) at 12:34:56:78:9a:bc [ether] on eth0
? (10.10.1.10) at <incomplete> on eth0
? (10.10.1.9) at 12:34:56:78:9a:bc [ether] on eth0

Routing table in 10.10.1.6

# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
168.63.129.16   10.10.1.1       255.255.255.255 UGH   0      0        0 eth0
10.10.1.0       0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth0
0.0.0.0         10.10.1.1       0.0.0.0         UG    0      0        0 eth0

Routing table in 10.10.1.9

 #route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
168.63.129.16   10.10.1.1       255.255.255.255 UGH   0      0        0 eth0
0.0.0.0         10.10.1.1       255.255.255.0   UG    0      0        0 eth0
10.10.1.0       0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth0
0.0.0.0         10.10.1.1       0.0.0.0         UG    0      0        0 eth0

Thanks in advance.

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6
  • I admit, I'm not familiar with the Azure cloud at all, however your statement "I have two VM's in azure with different public IP's" confuses me as the two IP addresses you listed are private IP's.
    – Mark Riddell
    May 27, 2016 at 7:43
  • Thanks for pointing out the confusing statement. Edited the post. I have not given the public ip's. i have just given the private ip's of server!!
    – Nilotpal
    May 27, 2016 at 8:34
  • Can you paste the output of the routing table on both VMs?
    – Mark Riddell
    May 27, 2016 at 8:40
  • added the in the post @MarkoPolo
    – Nilotpal
    May 27, 2016 at 8:43
  • 2
    The SYN is received by 10.10.1.9, but no SYN ACK is being returned. May I suggest you run netstat -natp | grep 2181 on 10.10.1.9 to make sure you actually have a listener on that port, and then try iptables -nvL OUTPUT to make sure there is no iptables rule blocking the return. If these are both successful, then the problem is likely with the telnet server configuration.
    – Jeremy Dover
    May 27, 2016 at 11:45

1 Answer 1

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3

I ran into a similar problem on RHEL7 and discovered that the host firewall was blocking the incoming connections. iptables showed an explicit rule to allow ssh access; other connections were blocked.

If you're on RHEL7 (or another OS using firewalld), try firewall-cmd --add-port=2181/tcp (or --add-service for a well-known port, like --add-service=https for HTTPS on port 443). If that clears it up, then tack on --permanent to make the setting persistent.

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