400 4xx Client error RFC 3261
SIP 400 Bad Request
The request could not be understood because of malformed syntax. The reason phrase often names the faulty header.
Typical Q.850 / ISDN cause: 41 (Temporary failure)
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Common causes
- Malformed or missing mandatory headers (Via, From, To, Call-ID, CSeq).
- Broken SDP, or bad characters in a display name.
- A SIP ALG on a router rewriting packets incorrectly.
- UDP fragmentation that cuts large messages.
How to fix it
- Capture the packet (sngrep or Wireshark) and validate the headers named in the reason phrase.
- Disable SIP ALG on firewalls and routers.
- Switch large messages to TCP, or trim unnecessary headers and codecs so the INVITE fits under the MTU.
Seeing 400 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.