Detect a transparent proxy
Many ISPs implement a transparent proxy for certain services (mostly http). Usually these proxies are not visible to the user so it is not possible determine if a connection problem is an issue with the remote host or the proxy. Tracing to ftp.cdrom.com on port 21 (ftp) returns a complete route to the server.
$ tracetcp ftp.cdrom.com:ftp
Tracing route to 207.250.14.6 on port 21
Over a maximum of 30 hops.
1 2 ms 1 ms 1 ms 192.168.0.1
2 9 ms 10 ms 8 ms 10.152.79.254
3 10 ms 33 ms 11 ms 62.253.122.137 [rdng-t2cam1-b-v102.inet.ntl.com]
4 27 ms 13 ms 10 ms 62.253.121.129 [winn-t2core-b-ge-wan61.inet.ntl.com]
5 12 ms 10 ms 9 ms 62.253.184.117 [win-bb-b-so-320-0.inet.ntl.com]
6 10 ms 11 ms 13 ms 62.253.185.201 [pop-bb-a-so-000-0.inet.ntl.com]
7 11 ms 9 ms 10 ms 212.113.14.129 [pos7-0.hsipaccess2.london1.level3.net]
8 11 ms 12 ms 11 ms 212.187.131.125 [so-4-0-0.mp2.london1.level3.net]
9 78 ms 76 ms 78 ms 212.187.128.153 [so-1-0-0.bbr2.newyork1.level3.net]
10 76 ms 75 ms 77 ms 64.159.17.104
11 85 ms 78 ms 93 ms 64.152.40.2 [us-nyc-c-r2-pos6-0.core.viatel.net]
12 78 ms 78 ms 81 ms 66.192.240.33
13 99 ms 97 ms 100 ms 168.215.53.41 [core-02-so-2-2-0-0.chcg.twtelecom.net]
14 123 ms 119 ms 118 ms 168.215.55.203 [dist-01-so-2-2-0-0.mpls.twtelecom.net]
15 119 ms 119 ms 119 ms 66.192.244.203 [hagg-01-ge-0-3-0-0.mpls.twtelecom.net]
16 119 ms 120 ms 120 ms 207.250.14.130
17 Destination Reached in 128 ms. Connection established to 207.250.14.6
Trace Complete.
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Performing the same trace on port 80 (http) shows the the packets take quite a different route after hop 2 and arrive at the proxy at hop 5. Note that the proxy responds to with the same IP address as the requested host.
$ tracetcp ftp.cdrom.com:http
Tracing route to 207.250.14.6 on port 80
Over a maximum of 30 hops.
1 1 ms 1 ms 1 ms 192.168.0.1
2 11 ms 11 ms 10 ms 10.152.79.254
3 10 ms 10 ms 9 ms 62.253.122.9 [rdng-t2cam1-a-v102.inet.ntl.com]
4 11 ms 11 ms 10 ms 62.253.121.1 [winn-t2core-a-ge-wan61.inet.ntl.com]
5 Destination Reached in 10 ms. Connection established to 207.250.14.6
Trace Complete.
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