ssh banner too large error

[yoousra]

Hi,

Used versions:

libnetconf2-3.7.10 libssh-0.10.5 netopeer2-2.4.5 sysrepo-3.7.11 libyang-3.13.6

I have a question about the banner leaf used in the callhome over ssh: https://github.com/CESNET/libnetconf2/blob/v3.7.10/modules/libnetconf2-netconf-server%402025-06-02.yang#L284

In the .yang it says the max length is 247 characters, so I fill it with 247, but the callhome 'connect' fails with error:

Starting the SSH session failed (Receiving banner: too large banner).

When I print the final content of the banner used in libssh code, I see that it starts with SSH-2.0- and then the content I filled the leaf banner with, and then potentially some other characters (maybe the Carriage Return (CR) and the single Line Feed (LF) as mentioned here: https://datatracker.ietf.org/doc/html/rfc4253#section-4.2 section 4.2 ?), with in total over 255 characters = SSH-2.0- + my_banner + CR + LF (maybe?) which gives 257 characters if we calculate the CR and LF if I understand well, which exceeds 255 characters, hence the error.

But this is just the hypothesis I made, so I wanted to make sure, does libnetconf2 add SSH-2.0- (and other characters?) to the banner I give?

Thank you for your clarification.

[Roytak]

Hi, libnetconf2 does not add anything. It would seem that libssh adds SSH-2.0- (8 bytes) to the front and \r\n (2 bytes) to the end of every banner. So you are right, the max length in YANG should be changed from 247 to length "1..245".