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.

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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