Paramiko : Error reading SSH protocol banner

Question

Recently, I made a code that connect to work station with different usernames (thanks to a private key) based on paramiko.

I never had any issues with it, but today, I have that : SSHException: Error reading SSH protocol banner

This is strange because it happens randomly on any connections. Is there any way to fix it ?

Answer 1

It depends on what you mean by "fix". The underlying cause, as pointed out in the comments, are congestion/lack of resources. In that way, it's similar to some HTTP codes. That's the normal cause, it could be that the ssh server is returning the wrong header data.

429 Too Many Requests, tells the client to use rate limiting, or sometimes APIs will return 503 in a similar way, if you exceed your quota. The idea being, to try again later, with a delay.

You can attempt to handle this exception in your code, wait a little while, and try again. You can also edit your transport.py file, to set the banner timeout to something higher. If you have an application where it doesn't matter how quickly the server responds, you could set this to 60 seconds.

EDIT: Editing your transport file is no longer needed as per Greg's answer. When you call connect, you can pass a banner_timeout (which solves this issue), a timeout (for the underlying TCP), and an auth_timeout (waiting for authentication response). Greg's answer has a code example with banner_timeout that you can directly lift.

Answer 2

Adding to TinBane's answers, suggesting to edit transport.py: you don't have to do that anymore.

Since Paramiko v. 1.15.0, released in 2015, (this PR, to be precise) you can configure that value when creating Paramiko connection, like this:

client = SSHClient()
client.connect('ssh.example.com', banner_timeout=200)

In the current version of Paramiko as of writing these words, v. 2.7.1, you have 2 more timeouts that you can configure when calling connect method, for these 3 in total (source):

• banner_timeout - an optional timeout (in seconds) to wait for the SSH banner to be presented.
• timeout - an optional timeout (in seconds) for the TCP connect
• auth_timeout - an optional timeout (in seconds) to wait for an authentication response.

Answer 3

When changing the timeout value (as TinBane mentioned) in the transport.py file from 15 to higher the issue resolved partially. that is at line #484:

self.banner_timeout = 200 # It was 15

However, to resolve it permanently I added a static line to transport.py to declare the new higher value at the _check_banner(self): function.

Here is specifically the change:

• It was like this:

 def _check_banner(self):
        for i in range(100):
            if i == 0:
                timeout = self.banner_timeout
            else:
                timeout = 2

• After the permanent change became like this:

 def _check_banner(self):
        for i in range(100):
            if i == 0:
                timeout = self.banner_timeout
                timeout = 200 # <<<< Here is the explicit declaration 
            else:
                timeout = 2

Answer 4

paramiko seems to raise this error when I pass a non-existent filename to kwargs>key_filename. I'm sure there are other situations where this exception is raised nonsensically.

Answer 5

I had this issue with 12 parallel (12 threads) connections via single bastion. As I had to solve it "quick and dirty" I've added a sleep time.

for target in targets:
    deployer.deploy_target(target, asynchronous=True)

Changed to:

for target in targets:
    deployer.deploy_target(target, asynchronous=True)
    time.sleep(5)

This works for me. As well I've added a banner_timeout as was suggested above to make it more reliable.

client.connect(bastion_address, banner_timeout=60)

Answer 6

I'm very new to this so I doubt I'm really qualified to answer anyone's questions, however I feel I may offer a simple solution to this issue.

After i migrated from 1 machine to another, all my scripts that worked perfectly prior all stopped working with the following error after the move:

Exception: Error reading SSH protocol banner
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/paramiko/transport.py", line 2211, in _check_banner

I tried as many people suggested above of manually amending the transport.py file but all that happened for me was it took 60 seconds to timeout rather than the default 15.

Anyway i noticed that my new machine was running a slightly older version on paramiko so I simply upgraded this and it worked.

pip3 -U paramiko

Answer 7

Well, I was also getting this with one of the juniper devices. The timeout didn't help at all. When I used pyex with this, it created multiple ssh/netconf sessions with juniper box. Once I changed the "set system services ssh connection-limit 10" From 5, it started working.

Answer 8

In my case, to speed up the connection download rate I have created a multiprocessing Pool of 10 processes, so 10 new ssh paramiko connections were active at the same time and downloading the data at the same time. When I increased this number to 20, I started getting this error. So, probably some congestion and too many connections active on one machine.