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I am trying to use ssh supporting GSSAPI with KeyExchange protocol with some servers that authenticate using kerberos. I have installed aur/openssh-gssapi. The problems is after getting the key from krb-server using kinit, when I ssh to the server it fails with this message:

Unable to negotiate with blah.blah: no matching key exchange method found. Their offer: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==

It says that they offer: [the key exchange above], but I have its support in my offerings:

debug1: Offering GSSAPI proposal: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-gex-sha1-A/vxljAEU54gt9a48EiANQ==,gss-group1-sha1-A/vxljAEU54gt9a48EiANQ==,gss-group14-sha1-A/vxljAEU54gt9a48EiANQ==,gss-gex-sha1-bontcUwnM6aGfWCP21alxQ==,gss-group1-sha1-bontcUwnM6aGfWCP21alxQ==,gss-group14-sha1-bontcUwnM6aGfWCP21alxQ==,gss-gex-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group1-sha1-eipGX3TCiQSrx573bT1o1Q==,gss-group14-sha1-eipGX3TCiQSrx573bT1o1Q==

I wonder what could be the cause of this problem!?

Edit: since I didn't see the point I did not post the log output of ssh command but just in case I am going to paste it in here:

debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: [email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,ssh-rsa
debug2: kex_parse_kexinit: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected],arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected],arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1,[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1,[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[email protected],zlib
debug2: kex_parse_kexinit: none,[email protected],zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: first_kex_follows 0
debug2: reserved 0
debug2: kex_parse_kexinit: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==
debug2: kex_parse_kexinit: null
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[email protected]
debug2: kex_parse_kexinit: none,[email protected]
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: first_kex_follows 0
debug2: reserved 0
debug1: kex: server->client [email protected] <implicit> none
debug1: kex: client->server [email protected] <implicit> none
Unable to negotiate with yada.yada: no matching key exchange method found. Their offer: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==
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  • Post the whole log and ideally the link to the gssapi patch that is applied in your package. It might be modified wrongly.
    – Jakuje
    Commented Oct 17, 2016 at 14:13
  • Well I think whoever uses archlinux knows how and what patches are applied to the openssh-server that I pointed at. The spec file is this: aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=openssh-gssapi
    – M.Rez
    Commented Oct 18, 2016 at 14:17
  • No, your client does not support any of GSSAPI kex algorithms: debug2: kex_parse_kexinit: [email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
    – Jakuje
    Commented Oct 18, 2016 at 14:23

1 Answer 1

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There is a bug in the patch in Arch Linux. They are setting the Kexs, but they are immediately overwritten by the other changes in the code:

The line

xasprintf(&myproposal[PROPOSAL_KEX_ALGS], "%s,%s", gss, orig);

should say

xasprintf(&options.kex_algorithms, "%s,%s", gss, orig);

From the brief check of the source against what we ship in Fedora.

Edit: Of course also this line:

orig = myproposal[PROPOSAL_KEX_ALGS];

should be

orig = options.kex_algorithms;
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  • Feel free to test and report the to the Arch maintainers. I am not familiar with their processes.
    – Jakuje
    Commented Oct 18, 2016 at 14:30
  • I am waiting for my account to be approved on their forum. I will report it as fast as it gets through. thanks. I will test it right away.
    – M.Rez
    Commented Oct 18, 2016 at 15:01
  • Not only I am very thankful but I am also very impressed how fast you found the problem.
    – M.Rez
    Commented Oct 18, 2016 at 19:00
  • Well ... this was one of more tricky parts during recent rebases of the same patch in Fedora so I quite remember what could go wrong if the author does not pay enough attention to the context :)
    – Jakuje
    Commented Oct 18, 2016 at 19:05

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