[2024-feb-29] Sad news: Eric Layton aka Nocturnal Slacker aka vtel57 passed away on Feb 26th, shortly after hospitalization. He was one of our Wiki's most prominent admins. He will be missed.
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howtos:misc:proxying_smpt [2015/11/27 22:27 (UTC)] – [Proxying Emails to Your Local Mail Relay Server] louigi600 | proxying_smtp [2016/06/20 07:21 (UTC)] – howtos:misc:proxying_smpt renamed to proxying_smtp (Typo in the page name) alienbob | ||
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As far as I know SMTP protocol does not support proxy itself but I can think of several ways to work around the problem. | As far as I know SMTP protocol does not support proxy itself but I can think of several ways to work around the problem. | ||
Here are some scenarios with examples on how you could go about working around the problem. | Here are some scenarios with examples on how you could go about working around the problem. | ||
- | Throughout the scenarios we will be referring so 3 machines: | ||
- | Machine A with IP address 192.168.0.2 (cannot send meils directly via internet and cannot use Local Mail Relay) | + | Throughout the scenarios we will be referring to 3 machines: |
- | Machine B with IP address 192.168.1.2 | + | * Machine A with IP address 192.168.0.2 (cannot send mails directly via internet and cannot relay on Local Mail Relay) |
- | + | * Machine B with IP address 192.168.1.2 | |
- | Local Mail Relay (LMR) with IP address 192.168.2.2 | + | |
===== Scenario 1 ===== | ===== Scenario 1 ===== | ||
- | Machine A cannot | + | * Machine A cannot |
- | but Machine A has access to machine | + | |
+ | * Machine B can relay thought LMR | ||
+ | * Machine B can reach LMR's port 25 | ||
- | We can use nc to proxy mail from A to LMR | + | We can use nc on Machine B to proxy mail from A to LMR. In this example it will appear to LMR as if it was Machine B that is sending the email. |
On machine B | On machine B | ||
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===== Scenario 2 ===== | ===== Scenario 2 ===== | ||
- | Let' | + | * Machine A cannot reach LMR' |
- | can we use a ssh tunnel between A and LMR to do the job for us ? | + | * Machine A is allowed to relay thought LMR but firewall is preventing it from reaching port 25 on LMR |
+ | * Machine A can reach the LMR but only on port 22. | ||
+ | |||
+ | We use a ssh tunnel between A and LMR to do the job for us. In this example we are bypassing firewall by using ssh to forward traffic trough a encrypted tunnel. For this to work properly the ssh client that is going to be forwarding needs to be executed by an administrative account. | ||
<note important> | <note important> | ||
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DATA | DATA | ||
354 End data with < | 354 End data with < | ||
- | FROM:drao@invallee.it | + | FROM:me@localnet.com |
- | TO:drao@invallee.it | + | TO:dude@localnet.com |
SUBJECT: | SUBJECT: | ||
if you get this sending email trough ssh tunnel worked | if you get this sending email trough ssh tunnel worked | ||
Line 81: | Line 85: | ||
+ | ===== Scenario 3 ===== | ||
+ | * Machine B can relay emails trough LMR | ||
+ | * Machine B cannot reach LMR's port 25 | ||
+ | * Machine B can access LMR via ssh | ||
+ | * Machine A has access to Machine B on whatever port we choose to let ssh listen on for forwarding (1024 in the exaple). | ||
- | This method may or may not work depending on whether Machine A is configured to be able use LMR for relaying mails (intendes as the MTA on LMR will allow Machien A to relay trought it). This leads to yet another scenario (see Scenario 3) or an alternative if Machine B can send emails trough | + | This is a bit like Scenario 1 but we will be using ssh to forward stuff to LMR and Machine B needs not be able to reach LMR's port 25 |
- | We can then start the ssh socks proxy on Machine B and have it listen | + | We can then start the ssh socks proxy on Machine B listening |
- | On machine B | + | On machine B with a privileged user |
ssh -2 -D 192.168.1.2: | ssh -2 -D 192.168.1.2: | ||
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DATA | DATA | ||
354 End data with < | 354 End data with < | ||
- | FROM:drao@invallee.it | + | FROM:me@localnet.com |
- | TO:drao@invallee.it | + | TO:dude@localnet.com |
SUBJECT: | SUBJECT: | ||
if you get this sending email trough ssh tunnel worked | if you get this sending email trough ssh tunnel worked | ||
Line 111: | Line 120: | ||
- | ===== Scenario | + | ===== Scenario |
- | Mostly like Scenario 2 but Machine A is not allowed to relay on LMR dew to MTA configuration. | + | * Machine A cannot reach LMR's port 25 |
+ | * Machine A is not allowed to relay thought | ||
+ | * Machine A cannot proxy on any other machine that is allowed | ||
+ | * Machine A can reach the LMR but only on port 22. | ||
We can still get the whole command to run on the LMR via ssh: | We can still get the whole command to run on the LMR via ssh: | ||
+ | |||
+ | <note important> | ||
cat << EOF > send_this_mail | cat << EOF > send_this_mail | ||
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quit | quit | ||
exit | exit | ||
- | | + | " | curl telnet:// |
+ | EOF | ||
cat send_this_mail |ssh 192.168.2.2 | cat send_this_mail |ssh 192.168.2.2 | ||
====== NOTES ====== | ====== NOTES ====== | ||
- | The command we used above is probably a good example of bad unixcraft, let me give a brief explanation on how it works. | + | The command we used in Scenario 1 is probably a good example of bad unixcraft, let me give a brief explanation on how it works. |
Let's momentarely get rid of the odd use of the fifo by just using this: | Let's momentarely get rid of the odd use of the fifo by just using this: | ||
Line 145: | Line 161: | ||
so now the whole command line looks like this: | so now the whole command line looks like this: | ||
cat /tmp/f | nc -l -p 1234 -s 192.168.1.2 | nc 192.168.2.2 25 >> /tmp/f | cat /tmp/f | nc -l -p 1234 -s 192.168.1.2 | nc 192.168.2.2 25 >> /tmp/f | ||
- | it is possible to do the same thing without having to use cat it is a little neater but still bad unixcraft | + | it is possible to do the same thing without having to use cat, it is a little neater but still bad unixcraft |
nc -l -p 1234 -s 192.168.1.2 < /tmp/f |nc 192.168.2.2 25 >> /tmp/f | nc -l -p 1234 -s 192.168.1.2 < /tmp/f |nc 192.168.2.2 25 >> /tmp/f | ||