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On one of the customer's locations I have a linux router that has been working fine for the last 500 days, but since yesterday, the internet from the internal network doesn't work and I can't SSH to the router. Pinging the WAN interface works. When I try to SSH to router it just waits. Running SSH in verbose mode yields the following:

debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/id_rsa type 1
debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-2048
debug1: Checking blacklist file /etc/ssh/blacklist.RSA-2048
debug1: identity file /root/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.7p1 Debian-12
debug1: match: OpenSSH_4.7p1 Debian-12 pat OpenSSH_4*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.1p1 Debian-5
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes256-ctr hmac-ripemd160 none
debug1: kex: client->server aes256-ctr hmac-ripemd160 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<4096<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP

And then it just waits there and nothing happens. The remote side eventually disconnects the connection. I have already instructed people there to restart the router, but it didn't change anything.

What could be the cause of it? How can I fix it? Going there is not possible for me so I will have to talk someone less techy through.

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When pinging the WAN interface, it works with package sizes up to 344B. There are no replies on ping requests when the size is 345B or more. I pinged from 2 different networks with the same results. – Matic Feb 4 '10 at 9:38

The network problem persisted when we changed the router for a windows machine and connected directly. Clearly the issue is not on our side but on the ISP's. Problem solving has been relayed to them.

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Since it was working until recently, I would guess a hardware issue. Of course it is also possible that someone hacked the box, but the other option is more likely.

I would replace it first, then do a post-mortem analysis, and fix it if possible.

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